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 as 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 



