28 VEGETABLE PHYSIOLOGY 
needed ; such forms as consist of single cells, or rows or 
plates of cells, living in water, need hardly any rigidity, 
and in their cases the unthickened cell-wall affords sufficient 
support to the protoplasm. Larger plants which grow in 
rapidly flowing water usually possess flexible stems and 
much-divided leaves, which consequently give way to the 
current and escape damage. Small terrestrial plants or 
parts of plants, which have but a short life, resemble these 
aquatic forms in their general characteristics, though they 
show much greater variety in the forms of their leaves. 
The rigidity and flexibility of both depend upon the disten- 
sion of their cells with water. We find this mechanism in 
succulent petioles, such ag those of the rhubarb, and in 
certain herbaceous stems which contain little wood, such 
as those of the cabbage and lettuce. Plants of terres- 
trial habit which attain very large dimensions, such as the 
forest trees, need, however, much greater modification. We 
have already studied how the outward form of their 
different parts, the root and the shoot, is adapted to their 
respective situations. Their internal structure also corre- 
sponds to their requirements, and helps to secure their safety. 
In many cases the strength and prominence of the 
tegumentary and conducting tissues supply the particular 
need of each part. In most forest trees we have seen that 
anchorage is afforded by the strong much-branched root 
system, the centre of whose members is composed of great 
developments of secondary wood, forming part of the 
conducting system. The trunk and twigs are of similar 
composition, the former being strengthened also by its bark. 
The distribution of the hard woody conducting tissue is 
very different in the subterranean and subaerial portions 
of the axis. In the former it is found to take the shape of 
a solid central core, a form which is well adapted to resist an 
uprooting pull, while it allows a considerable bending. In 
the subaerial portion as soon as the woody portion is 
developed it is found to be composed of several tough 
strands which, while the plant is young, are separate from 
