Page:British Flowering Plants.djvu/18

4 The stem is called sinuous when it curves backwards and forwards, jointed when it is interrupted at the nodes, as in Grasses, winding when it supports itself by other objects, as in Hops and Beans, climbing when it supports itself by thorns or aerial roots (Climbing Rose, or Ivy), clasping when it attaches itself by tendrils (Red Bryony, or Vine), erect when it rises straight, like the trunk of a tree, decumbent when it rests on the ground or rises obliquely (Mountain Pine), recumbent when it rests on the ground without rising (as in some species of Rubus), and creeping when it clings to the ground by roots (Strawberry).

It is not sufficient for the sustenance of plants merely to absorb water and other liquid nutriment through the roots, but it is also necessary for them to breathe air, and this is the principal function of the leaves. The green colour of plants is due to a peculiar substance called chlorophyll, which, under the influence of warmth and sunlight, absorbs from the surrounding air the gases required for the use of the plant. But plants (except fungi and other low plants, in which chlorophyll is not found) breathe in the opposite way to animals, for animals absorb oxygen to aerate their blood, and exhale carbonic acid, which, in quantity, is poisonous to them; whereas plants require carbon for building up their tissues, and therefore absorb the carbonic acid from the air, and exhale oxygen. Thus, during the day, plants greatly assist in purifying the air which animals breathe; but at night this process is somewhat reversed, for while it is necessary to their life for animals to breathe day and night, in plants the function of chlorophyll ceases at night, and any superfluity of carbonic acid which has been inhaled during the day is then exhaled. Consequently, it is not healthy to allow many plants to remain in a sleeping-room at night. But just as chlorophyll is wanting in some low groups of plants, so is it present in the Hydra and in some other low forms of animal life.

Leaves vary much in form, and are usually attached to a branch or stem by a stalk or petiole.