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The transformed leaves
In many species of plants the leaves bring to the right and left of the base of the petiole or of the lamina two extroflexions of variable size and shape called stipules (the stipules can in turn be transformed into thorns as happens for Robinia). Tendrils, petals, sepals and stamens are leaves that have undergone profound transformations.
Some climbing species have modified leaves, called tendrils (or cirrus), to perform the function of support. Unlike leaves that have limited growth (when development is complete they stop their growth), the tendrils have indefinite growth (they possess unlimited activity meristems). The term tendril does not refer exclusively to modified leaves: in species such as the grape vine and the American vine the tendrils did not originate from the leaves, but rather from the branches.
Many species of arid environments have non-photosynthesizing leaves, with mechanical defense function, called thorns; the photosynthetic function is entrusted to the stem, which is therefore green in color. In these species, the stem also performs the function of organ for the conservation of water; plants that have specialized fabrics for this function are called succulents. In the absence of adequate mechanical defense, in the environment in which they live, where water and food are very scarce, these plants would be devoured by herbivores.
Spine-like structures, but with different evolutionary origins, are the spines and emergencies: the former are modified branches while the latter are protuberances of the bark.
The dormancy of the buds is of primary importance for the survival of perennials of climates characterized by an unfavorable season (represented, in temperate climates, by winter). The dormant bud is an embryonic bud consisting of an apical meristem, a series of nodes and internodes not yet elongated, various rudimentary leaves, primordial leaves and modified leaves, called perules (or scales of the buds) that envelop the whole. The perulas act as thermal insulators and help prevent drying, as well as reduce the penetration of water and the circulation of oxygen in the bud. Since the main function of the perules is that of protection, they are not synthetically active photos, they have a short or completely absent petiole, they are very rigid with a frequently leathery consistency and rich, especially on the face exposed to the atmosphere, in hydrophobic substances (suberine , resins and waxes).
Leaves with a reserve function of nutrients
In the bulb of some biennial species, such as onion, fleshy leaves assume the function of reserve organs. These modified leaves, called catafilli, accumulate carbohydrates, used during the vegetative restart at the end of the winter season.
In many succulent plants the water conservation function is entrusted to the leaves. The most evident morphological modification of the succulent leaves is given by the increase in thickness, which entails a strong reduction in the surface / volume ratio and consequently a lower loss of water through perspiration. This causes, however, a decrease in the amount of carbon dioxide absorbed by the leaf; moreover, only the layers of parenchyma closest to the leaf surface are reached by light and therefore photosynthetically active, while the deep ones perform exclusively the function of water reserve. It is therefore evident that water saving occurs at the expense of photosynthetic activity.
Leaves of insectivorous plants
Insectivorous plants supplement their "diet" (consisting of water, minerals and organic substances absorbed through the roots) with animals, captured and digested through special modified leaves. The places where these plants live (swamps, bare rocks, trunks of other plants, submerged environments), are poor in nitrogen substances, which carnivorous plants can obtain by digesting their prey.