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 



