24 VEGETABLE PHYSIOLOGY 
both depend upon the distension 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 terrestrial habit which attain very 
large dimensions, such as the forest trees, need, however, 
much greater modification. Being exposed to winds and 
storms, they need a firm anchorage below the surface of 
the ground, and a more or less massive axis to secure 
stability when atmospheric disturbances are severe. For 
the needs of their protoplasts, to secure the exposure of 
the greatest possible number of them to the access of air, 
warmth, and light, a great subdivision of this axis is 
necessary, so that the form usually attained is that of a 
relatively very large head resulting from the repeated sub- 
divisions of the trunk, and ending in finely divided twigs. 
The danger of too great rigidity in this portion soon 
becomes apparent, as it presents a very considerable 
surface to the wind. The rigidity needed for support must 
be combined with sufficient flexibility to enable the body, 
already helped by its fine subdivision, to give way before 
the force of atmospheric currents, and so to prevent the 
danger of uprooting the tree. In other forms a weak axis, 
quite incapable of supporting any great development of the 
plant-body, must be capable of obtaining support by cling- 
ing in various ways, and holding by various mechanisms 
to other structures, such as the trunks of trees, rocks, 
walls, &c. 
In many cases the strength and prominence of the 
tegumentary and conducting tissues supply the particular 
need. In most forest trees the 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 very materially also by its bark. But there 
are many smaller trees and shrubby plants, as well as some 
