The flowering plants known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families 13,164 known genera and c. 369,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, the production of fruits that contain the seeds. Etymologically, angiosperm means a plant; the term comes from the Greek words sperma. The ancestors of flowering plants diverged from gymnosperms in the Triassic Period, 245 to 202 million years ago, the first flowering plants are known from 160 mya, they diversified extensively during the Early Cretaceous, became widespread by 120 mya, replaced conifers as the dominant trees from 100 to 60 mya. Angiosperms differ from other seed plants in several ways, described in the table below; these distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans.
Angiosperm stems are made up of seven layers. The amount and complexity of tissue-formation in flowering plants exceeds that of gymnosperms; the vascular bundles of the stem are arranged such that the phloem form concentric rings. In the dicotyledons, the bundles in the young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles, a complete ring is formed, a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside; the soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.
Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They once formed the stem increases in diameter only in exceptional cases; the characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, provide the most trustworthy external characteristics for establishing relationships among angiosperm species; the function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally from the axil of a leaf; as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More the flower-bearing portion of the plant is distinguished from the foliage-bearing or vegetative portion, forms a more or less elaborate branch-system called an inflorescence. There are two kinds of reproductive cells produced by flowers. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens.
The "female" cells called megaspores, which will divide to become the egg cell, are contained in the ovule and enclosed in the carpel. The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Other structures are present and serve to protect the sporophylls and to form an envelope attractive to pollinators; the individual members of these surrounding structures are known as petals. The outer series is green and leaf-like, functions to protect the rest of the flower the bud; the inner series is, in general, white or brightly colored, is more delicate in structure. It functions to attract bird pollinators. Attraction is effected by color and nectar, which may be secreted in some part of the flower; the characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans. While the majority of flowers are perfect or hermaphrodite, flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization.
Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot transfer pollen to the pistil. Homomorphic flowers may employ a biochemical mechanism called self-incompatibility to discriminate between self and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers; the botanical term "Angiosperm", from the Ancient Greek αγγείον, angeíon and σπέρμα, was coined in the form Angiospermae by Paul Hermann in 1690, as the name of one of his primary divisions of the plant kingdom. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked; the term and its antonym were maintained by Carl Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any
Phytochemistry is the study of phytochemicals, which are chemicals derived from plants. Those studying phytochemistry strive to describe the structures of the large number of secondary metabolic compounds found in plants, the functions of these compounds in human and plant biology, the biosynthesis of these compounds. Plants synthesize phytochemicals for many reasons, including to protect themselves against insect attacks and plant diseases. Phytochemicals in food plants are active in human biology, in many cases have health benefits; the compounds found in plants are of many kinds, but most are in four major biochemical classes, the alkaloids, glycosides and terpenes. Phytochemistry can be considered sub-fields of chemistry. Activities can be led in the wild with the aid of ethnobotany; the applications of the discipline can be for pharmacognosy, or the discovery of new drugs, or as an aid for plant physiology studies. Techniques used in the field of phytochemistry are extraction and structural elucidation of natural products, as well as various chromatography techniques.
The list of simple elements of which plants are constructed—carbon, hydrogen, phosphorus, etc.—is not different from similar lists for animals, fungi, or bacteria. The fundamental atomic components of plants are the same as for all life. Phytochemistry is used in the field of Chinese medicine in the field of herbal medicine. Phytochemical technique applies to the quality control of Chinese medicine, Ayurvedic medicine or herbal medicine of various chemical components, such as saponins, volatile oils and anthraquinones. In the development of rapid and reproducible analytical techniques, the combination of HPLC with different detectors, such as diode array detector, refractive index detector, evaporative light scattering detector and mass spectrometric detector, has been developed. In most cases, biologically active compounds in Chinese medicine, Ayurveda, or herbal medicine have not been determined. Therefore, it is important to use the phytochemical methods to screen and analyze bioactive components, not only for the quality control of crude drugs, but for the elucidation of their therapeutic mechanisms.
Modern pharmacological studies indicate that binding to receptors or ion channels on cell membranes is the first step of some drug actions. A new method in phytochemistry called biochromatography has been developed; this method combines human red cell membrane extraction and high performance liquid chromatography to screen potential active components in Chinese medicine. Many plants produce chemical compounds for defence against herbivores; these are useful as drugs, the content and known pharmacological activity of these substances in medicinal plants is the scientific basis for their use. The major classes of pharmacologically active phytochemicals are described below, with examples of medicinal plants that contain them. Human settlements are surrounded by weeds useful as medicines, such as nettle and chickweed. Many phytochemicals, including curcumin, epigallocatechin gallate and resveratrol are pan-assay interference compounds and are not useful in drug discovery. Alkaloids are bitter-tasting chemicals widespread in nature, toxic.
There are several classes with different modes of action as drugs, both recreational and pharmaceutical. Medicines of different classes include atropine and hyoscyamine, the traditional medicine berberine, cocaine, morphine, reserpine and quinine, vincristine. Anthraquinone glycosides are found in the laxatives senna and Aloe; the cardiac glycosides are powerful drugs from plants including lily of the valley. They include digoxin and digitoxin which support the beating of the heart, act as diuretics. Polyphenols of several classes are widespread in plants, they include the colourful anthocyanins, hormone-mimicking phytoestrogens, astringent tannins. In Ayurveda, the astringent rind of the pomegranate is used as a medicine, while polyphenol extracts from plant materials such as grape seeds are sold for their potential health benefits They have been continually studied in cell cultures for their different anti-cancer effects. Plants containing phytoestrogens have been used for centuries to treat gynaecological disorders such as fertility and menopausal problems.
Among these plants are Pueraria mirifica, angelica and anise. Terpenes and terpenoids of many kinds are found in resinous plants such as the conifers, they are aromatic and serve to repel herbivores. Their scent makes them useful in essential oils, whether for perfumes such as rose and lavender, or for aromatherapy; some have had medicinal uses: thymol is an antiseptic and was once used as a vermifuge. Tropical Botanical Garden and Research Institute UBC Botanical Garden and Centre for Plant Research
A pteridophyte is a vascular plant that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are referred to as "cryptogams", meaning that their means of reproduction is hidden; the pteridophytes include the ferns and the lycophytes. These are not a monophyletic group because ferns and horsetails are more related to seed plants than to the lycophytes. Therefore, "Pteridophyta" is no longer a accepted taxon, although the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, to indicate lycophytes and ferns as an informal grouping, such as the International Association of Pteridologists and the Pteridophyte Phylogeny Group. Ferns and lycophytes are free-sporing vascular plants that have a life cycle with free-living, independent gametophyte and sporophyte phases, their other common characteristics include vascular plant apomorphies and land plant plesiomorphies. Of the pteridophytes, ferns account for nearly 90% of the extant diversity.
Smith et al. the first higher-level pteridophyte classification published in the molecular phylogenetic era, considered the ferns as monilophytes, as follows: Division Tracheophyta - vascular plants Sub division Euphyllophytina Infradivision Moniliformopses Infradivision Spermatophyta - seed plants, ~260,000 species Subdivision Lycopodiophyta - less than 1% of extant vascular plantswhere the monilophytes comprise about 9,000 species, including horsetails, whisk ferns, all eusporangiate and all leptosporangiate ferns. Both lycophytes and monilophytes were grouped together as pteridophytes on the basis of being spore-bearing. In Smith's molecular phylogenetic study the ferns are characterised by lateral root origin in the endodermis mesarch protoxylem in shoots, a pseudoendospore, plasmodial tapetum, sperm cells with 30-1000 flagella; the term "moniliform" as in Moniliformopses and monilophytes means "bead-shaped" and was introduced by Kenrick and Crane as a scientific replacement for "fern" and became established by Pryer et al..
Christenhusz and Chase in their review of classification schemes provide a critique of this usage, which they discouraged as irrational. In fact the alternative name Filicopsida was in use. By comparison "lycopod" or lycophyte means wolf-plant; the term "fern ally" included under Pteridophyta refers to vascular spore-bearing plants that are not ferns, including lycopods, whisk ferns and water ferns, a much wider range of taxa. This is not a natural grouping but rather a convenient term for non-fern, is discouraged, as is eusporangiate for non-leptosporangiate ferns; however both Infradivision and Moniliformopses are invalid names under the International Code of Botanical Nomenclature. Ferns, despite forming a monophyletic clade, are formally only considered as four classes, 11 orders and 37 families, without assigning a higher taxonomic rank. Furthermore, within the Polypodiopsida, the largest grouping, a number of informal clades were recognised, including leptosporangiates, core leptosporangiates and eupolypods.
In 2014 Christenhusz and Chase, summarising the known knowledge at that time, treated this group as two separate unrelated taxa in a consensus classification. 1,300 species Polypodiophyta 4 sublasses, 11 orders, 21 families, approx. 212 genera, approx. 10,535 species Subclass Equisetidae Warm. Subclass Ophioglossidae Klinge Subclass Marattiidae Klinge Subclass Polypodiidae Cronquist, Takht. & Zimmerm. These subclasses correspond to Smith's four classes, with Ophioglossidae corresponding to Psilotopsida; the two major groups included in Pteridophyta are phylogenetically related as follows: Pteridophytes consist of two separate but related classes, whose nomenclature has varied. The terminology used by the Pteridophyte Phylogeny Group is used here: Classes and ordersLycopodiopsida Lycopodiidae Selaginellidae Polypodiopsida Equisetidae Ophioglossidae Psilotales Ophioglossales Marattiidae Polypodiidae In addition to these living groups, several groups of pteridophytes are now extinct and known only from fossils.
These groups include the Rhyniopsida, Zosterophyllopsida, Trimerophytopsida, the Lepidodendrales and the Progymnospermopsida. Modern studies of the land plants agree that all pteridophytes share a common ancestor with seed plants. Therefore, pteridophytes constitute a paraphyletic group. Just as with seed plants and mosses, the life cycle of pteridophytes involves alternation of generations; this means. Pteridophytes differ from mosses and seed plants in that both generations are independent and free-living, although the sporophyte is much larger and more conspicuous; the sexuality of pteridophyte gametophytes can be classifi
A storage organ is a part of a plant modified for storage of energy or water. Storage organs grow underground, where they are better protected from attack by herbivores. Plants that have an underground storage organ are called geophytes in the Raunkiær plant life-form classification system. Storage organs but not always, act as perennating organs which enable plants to survive adverse conditions. Storage organs may act as perennating organs; these are used by plants to survive adverse periods in the plant's life-cycle. During these periods, parts of the plant die and when conditions become favourable again, re-growth occurs from buds in the perennating organs. For example, geophytes growing in woodland under deciduous trees die back to underground storage organs during summer when tree leaf cover restricts light and water is less available. However, perennating organs need not be storage organs. After losing their leaves, deciduous trees grow them again from'resting buds', which are the perennating organs of phanerophytes in the Raunkiær classification, but which do not act as storage organs.
Storage organs need not be perennating organs. Many succulents have leaves adapted for water storage. In common parlance, underground storage organs may be generically called roots, tubers, or bulbs, but to the botanist there is more specific technical nomenclature: True roots: Tuberous root or root tuber — e.g. Dahlia Storage taproot — e.g. carrot Modified stems: Corm — e.g. Crocus Stem tuber — e.g. Zantedeschia, potato Rhizome — e.g. Iris pseudacorus Pseudobulb — e.g. Pleione Caudex — e.g. Adenium Others: Storage hypocotyl — sometimes called a tuber, as in Cyclamen Bulb — e.g. Lilium, onionSome of the above pseudobulbs and caudices, may occur wholly or above ground. Intermediates and combinations of the above are found, making classification difficult; as an example of an intermediate, the tuber of Cyclamen arises from the stem of the seedling, which forms the junction of the roots and stem of the mature plant. In some species roots come from the bottom of the tuber; as an example of a combination, juno irises have both bulbs and storage roots.
Underground storage organs used for food may be generically called root vegetables, although this phrase should not be taken to imply that the class only includes true roots. Succulents are plants which are adapted to withstand periods of drought by their ability to store moisture in specialized storage organs. Leaf succulents store water in their leaves, which are thus thickened and covered with a waxy coating or fine hairs to reduce evaporation, they may contain mucilaginous compounds. Some leaf succulents have leaves which are distributed along the stem in a similar fashion to non-succulent species. In others, the leaves are more compact. Pebble-plants or living stones have reduced their leaves to just two, forming a fleshy body, only the top of which may be visible above ground. Stem succulents are either leafless or have leaves which can be shed in the event of drought. Photosynthesis is taken over by the stems; as with leaf succulents, stems may be covered with a waxy coating or fine hairs to reduce evaporation.
The ribbed bodies of cacti may be an adaption to allow shrinkage and expansion with the amount of water stored. Plants of the same general form as cacti are found in other families
Monocotyledons referred to as monocots, are flowering plants whose seeds contain only one embryonic leaf, or cotyledon. They constitute one of the major groups into which the flowering plants have traditionally been divided, the rest of the flowering plants having two cotyledons and therefore classified as dicotyledons, or dicots. However, molecular phylogenetic research has shown that while the monocots form a monophyletic group or clade, the dicots do not. Monocots have always been recognized as a group, but with various taxonomic ranks and under several different names; the APG III system of 2009 recognises a clade called "monocots" but does not assign it to a taxonomic rank. The monocots include about 60,000 species; the largest family in this group by number of species are the orchids, with more than 20,000 species. About half as many species belong to the true grasses, which are economically the most important family of monocots. In agriculture the majority of the biomass produced; these include not only major grains, but forage grasses, sugar cane, the bamboos.
Other economically important monocot crops include various palms and plantains, gingers and their relatives and cardamom, pineapple, water chestnut, leeks and garlic. Many houseplants are monocot epiphytes. Additionally most of the horticultural bulbs, plants cultivated for their blooms, such as lilies, irises, cannas and tulips, are monocots; the monocots or monocotyledons have, as the name implies, a single cotyledon, or embryonic leaf, in their seeds. This feature was used to contrast the monocots with the dicotyledons or dicots which have two cotyledons. From a diagnostic point of view the number of cotyledons is neither a useful characteristic, nor is it reliable; the single cotyledon is only one of a number of modifications of the body plan of the ancestral monocotyledons, whose adaptive advantages are poorly understood, but may have been related to adaption to aquatic habitats, prior to radiation to terrestrial habitats. Monocots are sufficiently distinctive that there has been disagreement as to membership of this group, despite considerable diversity in terms of external morphology.
However, morphological features that reliably characterise major clades are rare. Thus monocots are distinguishable from other angiosperms both in terms of their uniformity and diversity. On the one hand the organisation of the shoots, leaf structure and floral configuration are more uniform than in the remaining angiosperms, yet within these constraints a wealth of diversity exists, indicating a high degree of evolutionary success. Monocot diversity includes perennial geophytes such as ornamental flowers including and succulent epiphytes, all in the lilioid monocots, major cereal grains in the grass family and forage grasses as well as woody tree-like palm trees, bamboo and bromeliads, bananas and ginger in the commelinid monocots, as well as both emergent and aroids, as well as floating or submerged aquatic plants such as seagrass. Organisation and life formsThe most important distinction is their growth pattern, lacking a lateral meristem that allows for continual growth in diameter with height, therefore this characteristic is a basic limitation in shoot construction.
Although herbaceous, some arboraceous monocots reach great height and mass. The latter include agaves, palms and bamboos; this creates challenges in water transport. Some, such as species of Yucca, develop anomalous secondary growth, while palm trees utilise an anomalous primary growth form described as establishment growth; the axis undergoes primary thickening, that progresses from internode to internode, resulting in a typical inverted conical shape of the basal primary axis. The limited conductivity contributes to limited branching of the stems. Despite these limitations a wide variety of adaptive growth forms has resulted from epiphytic orchids and bromeliads to submarine Alismatales and mycotrophic Burmanniaceae and Triuridaceae. Other forms of adaptation include the climbing vines of Araceae which use negative phototropism to locate host trees, while some palms such as Calamus manan produce the longest shoots in the plant kingdom, up to 185 m long. Other monocots Poales, have adopted a therophyte life form.
LeavesThe cotyledon, the primordial Angiosperm leaf consists of a proximal leaf base or hypophyll and a distal hyperphyll. In monocots the hypophyll tends to be the dominant part in contrast to other angiosperms. From these, considerable diversity arises. Mature monocot leaves are narrow and linear, forming a sheath
Plant ecology is a subdiscipline of ecology which studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, the interactions among and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands. A global overview of the Earth's major vegetation types is provided by O. W. Archibold, he recognizes 11 major vegetation types: tropical forests, tropical savannas, arid regions, Mediterranean ecosystems, temperate forest ecosystems, temperate grasslands, coniferous forests, terrestrial wetlands, freshwater ecosystems and coastal/marine systems. This breadth of topics shows the complexity of plant ecology, since it includes plants from floating single-celled algae up to large canopy forming trees. One feature that defines plants is photosynthesis.
Photosynthesis is the process of a chemical reactions to create glucose and oxgyen, vital for plant life. One of the most important aspects of plant ecology is the role plants have played in creating the oxygenated atmosphere of earth, an event that occurred some 2 billion years ago, it can be dated by the deposition of banded iron formations, distinctive sedimentary rocks with large amounts of iron oxide. At the same time, plants began removing carbon dioxide from the atmosphere, thereby initiating the process of controlling Earth's climate. A long term trend of the Earth has been toward increasing oxygen and decreasing carbon dioxide, many other events in the Earth's history, like the first movement of life onto land, are tied to this sequence of events. One of the early classic books on plant ecology was written by J. E. Weaver and F. E. Clements, it talks broadly about plant communities, the importance of forces like competition and processes like succession. Plant ecology can be divided by levels of organization including plant ecophysiology, plant population ecology, community ecology, ecosystem ecology, landscape ecology and biosphere ecology.
The study of plants and vegetation is complicated by their form. First, most plants are rooted in the soil, which makes it difficult to observe and measure nutrient uptake and species interactions. Second, plants reproduce vegetatively, asexually, in a way that makes it difficult to distinguish individual plants. Indeed, the concept of an individual is doubtful, since a tree may be regarded as a large collection of linked meristems. Hence, plant ecology and animal ecology have different styles of approach to problems that involve processes like reproduction and mutualism; some plant ecologists have placed considerable emphasis upon trying to treat plant populations as if they were animal populations, focusing on population ecology. Many other ecologists believe that while it is useful to draw upon population ecology to solve certain scientific problems, plants demand that ecologists work with multiple perspectives, appropriate to the problem, the scale and the situation. Plant ecology has its origin in the application of plant physiology to the questions raised by plant geographers.
Carl Ludwig Willdenow was one of the first to note that similar climates produced similar types of vegetation when they were located in different parts of the world. Willdenow's student, Alexander von Humboldt, used physiognomy to describe vegetation types and observed that the distribution vegetation types was based on environmental factors. Plant geographers who built upon Humboldt's work included Joakim Frederik Schouw, A. P. de Candolle, August Grisebach and Anton Kerner von Marilaun. Schouw's work, published in 1822, linked plant distributions to environmental factors and established the practice of naming plant associations by adding the suffix -etum to the name of the dominant species. Working from herbarium collections, De Candolle searched for general rules of plant distribution and settled on using temperature as well. Grisebach's two-volume work, Die Vegetation der Erde nach Ihrer Klimatischen Anordnung, published in 1872, saw plant geography reach its "ultimate form" as a descriptive field.
Starting in the 1870s, Swiss botanist Simon Schwendener, together with his students and colleagues, established the link between plant morphology and physiological adaptations, laying the groundwork for the first ecology textbooks, Eugenius Warming's Plantesamfund and Andreas Schimper's 1898 Pflanzengeographie auf Physiologischer Grundlage. Warming incorporated plant morphology, physiology taxonomy and biogeography into plant geography to create the field of plant ecology. Although more morphological than physiological, Schimper's has been considered the beginning of plant physiological ecology. Plant ecology was built around static ideas of plant distribution. Henry Chandler Cowles' studies of plant succession on the Lake Michigan sand dunes and Frederic Clements' 1916 monograph on the subject established it as a key element of plant ecology. Plant ecology developed within the wider discipline of ecology over the twentieth century. Inspired by Warming's Plantesamfund, Arthur Tansley set out to map British plant communities.
In 1904 he teamed up with William Gardner Smith and others involved in vegetation mapping to establish the Central Committee for the Survey and Study of British Vegetation shortened to British Vegetation Committee. In 1913, the British Vegetation Committee organised the British Ecological
A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage". A leaf is a thin, dorsiventrally flattened organ borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinct upper surface and lower surface that differ in colour, the number of stomata, the amount and structure of epicuticular wax and other features. Leaves can have many different shapes and textures; the broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In the clubmosses, with different evolutionary origins, the leaves are simple and are known as microphylls.
Some leaves, such as bulb scales, are not above ground. In many aquatic species the leaves are submerged in water. Succulent plants have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls and spines. Furthermore, several kinds of leaf-like structures found in vascular plants are not homologous with them. Examples include flattened plant stems called phylloclades and cladodes, flattened leaf stems called phyllodes which differ from leaves both in their structure and origin; some structures of non-vascular plants function much like leaves. Examples include the phyllids of liverworts. Leaves are the most important organs of most vascular plants. Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis, they capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolised by cellular respiration to provide chemical energy to run cellular processes.
The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf, while leaves are orientated to maximise their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots. Vascular plants transport sucrose in a special tissue called the phloem; the phloem and xylem are parallel to each other but the transport of materials is in opposite directions. Within the leaf these vascular systems branch to form veins which supply as much of the leaf as possible, ensuring that cells carrying out photosynthesis are close to the transportation system. Leaves are broad and thin, thereby maximising the surface area directly exposed to light and enabling the light to penetrate the tissues and reach the chloroplasts, thus promoting photosynthesis.
They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. For instance plants adapted to windy conditions may have pendent leaves, such as in many willows and eucalyptss; the flat, or laminar, shape maximises thermal contact with the surrounding air, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of transpiration, providing the energy required to draw the transpiration stream up from the roots, guttation. Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost; these are interpreted as reduced from megaphyllous leaves of their Devonian ancestors. Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favour of protection from herbivory.
For xerophytes the major constraint drought. Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. and Bulbine mesembryanthemoides. Leaves function to store chemical energy and water and may become specialised organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia. Leaves are the fundamental structural units from which cones are constructed in gymnosperms and from which flowers are constructed in flowering plants; the internal organisation of most kinds of leaves has evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide while at the same time controlling water loss. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute openings called stomata which open or close to regulate the rate exchange of carbon dioxide and water vapour into