Cycas

A leaf (pl.: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, flower, and fruit collectively form the shoot system. In most leaves, the primary photosynthetic tissue is the palisade mesophyll and 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. The leaf is an integral part of the stem system, and most leaves are flattened and have distinct upper (adaxial) and lower (abaxial) surfaces that differ in color, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax, and other features. Leaves are mostly green in color due to the presence of a compound called chlorophyll which is essential for photosynthesis as it absorbs light energy from the Sun. A leaf with lighter-colored or white patches or edges is called a variegated leaf.

Leaves vary in shape, size, texture and color, depending on the species 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, which also include acrogymnosperms and ferns. In the lycopods, with different evolutionary origins, the leaves are simple (with only a single vein) 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 often 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 totally homologous with them. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin. Some structures of non-vascular plants look and function much like leaves. Examples include the phyllids of mosses and liverworts.

A leaf (pl.: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, flower, and fruit collectively form the shoot system. In most leaves, the primary photosynthetic tissue is the palisade mesophyll and 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. The leaf is an integral part of the stem system, and most leaves are flattened and have distinct upper (adaxial) and lower (abaxial) surfaces that differ in color, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax, and other features. Leaves are mostly green in color due to the presence of a compound called chlorophyll which is essential for photosynthesis as it absorbs light energy from the Sun. A leaf with lighter-colored or white patches or edges is called a variegated leaf.

Leaves vary in shape, size, texture and color, depending on the species. 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, which also include acrogymnosperms and ferns. In the lycopods, with different evolutionary origins, the leaves are simple (with only a single vein) 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 often 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 totally homologous with them. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin. Some structures of non-vascular plants look and function much like leaves. Examples include the phyllids of mosses and liverworts.

Pachycauls are plants with a disproportionately thick trunk, for their height, and relatively few branches. With certain pachycaul species, particularly the more succulent varieties, they are commonly referred to as "caudiciformes", a reference to their trunk development of a moisture-filled caudex for periods of drought. By comparison, trees with thin twigs, such as oaks (Quercus), maples (Acer) and Eucalyptus, are called leptocauls; those with moderately thick twigs, such as Plumeria, are called mesocauls. Pachycauls can be the product of exceptional primary growth (as with Arecaceae and Cycadaceae) or disproportionate secondary growth, as with the baobabs (Adansonia). The word is derived from the Greek pachy- (meaning "thick" or "stout") and the Latin caulis (meaning "stem").

All of the arboreal (treelike) species of Cactaceae are pachycauls, as are most of the Arecaceae, Cycadaceae and Pandanus. The most extreme pachycauls are the floodplains, or river-bottom, varieties of the African palmyra (Borassus aethiopum), with primary growth up to 2.1 m (6 ft 11 in) thick, and the Coquito palm (Jubaea chilensis), with its primary growth measuring up to 1.8 m (5 ft 11 in) thick. The most pachycaulous cycad is Cycas thouarsii at up to 150 cm (59 in) diameter. The tallest pachycaul is the Andean wax palm (Ceroxylon quindiuense), at heights of up to 66 m (217 ft), and around 41 cm (16 in) in diameter. The most pachycaulous cactus are the barrel cacti (Echinocactus platyacanthus), with primary growth up to 1.3 m (4 ft 3 in) diameter. The largest caudiciforme-type pachycaul is the African baobab (Adansonia digitata); one specimen of which, called the Glencoe baobab (from Hoedspruit, Limpopo, South Africa) has a basal diameter (not girth) of 16 m (52 ft). However, this particular tree has suffered a severe trauma and is dying. Pachycauls also differ greatly in their rates of growth, from the half-man tree (Pachypodium namaquanum) which grows only 5 mm (0.20 in) each year to the palm Pigafetta elata which can grow 2.7 m (8 ft 10 in) or more in one year.