When Plants Don’t Get Enough Water
ADAPTATIONS OF LEAVES
Against the ever-present danger of unsatisfied thirst and subsequent drying out, many plants have evolved some kind of safeguard.
Most plants have their stomata on the underside of their leaves, away from the drying rays of the sun. Often when the water content of the leaf falls, the leaf curls or folds inward to close the stomata and check transpiration losses.
Many plants, such as thyme, heathers and coniferous trees, living habitually in exposed conditions, have small leaves to reduce transpiration losses. Others, such as Stachys lanata (lamb’s ear), have a covering of hair or silky down on their leaves to retard the drying effect of wind; others, such as the hollies, grow shiny, leathery leaves with fewer stomata, and others, such as primulas, develop waxy or mealy coatings to retard evaporation. Plants native to hot, dry conditions often reduce their leaves to stems, as in the, or develop thickened leaves with capacity for water storage, as in such as houseleeks. On the other hand, plants that grow in damp places, such as palustris (marsh marigold), tend to have large soft leaves, well equipped with stomata to allow them to transpire freely.
In warm regions where there is a continuous supply of water, leaves may carry on their work all the year round. Plants in these regions do not shed their leaves, but tend to become evergreen.
Much of the variation in the shape, size, texture and appearance of leaves, that lends so much beauty to the garden, is dictated by the plants’ water requirement.
Where there is a season of cold, drying winds, such as an English winter, or a season of drought, a plant loses more moisture than it can make good. Many plants adapt themselves to such conditions by storing up reserves of food, forming latent or dormant buds, and dropping their leaves. The trees and shrubs that do this are called deciduous.
Before falling, the green chlorophyll in their leaves decomposes, often allowing the hitherto masked bright pigments to show and make a blaze of autumn colour. Then the leaves are pushed off and the framework of the plant, with its reserves of food, can weather the adverse season.
adapt them-selves by losing all their top growth, and storing food reserves below ground in their roots or, as in the case of bulbs, corms and rhizomes, in what are swollen leaves or stems.