22 VEGETABLE PHYSIOLOGY 



a ready means of communication must die in consequence 

 of their position. These moreover are among the most 

 active of the protoplasts, discharging important duties in 

 connection with nutrition, and needing for their purpose 

 considerable quantities of the water from the soil with the 

 salts dissolved in it. 



The main conducting system is formed by the collections 

 of cells and vessels which are known as the vascular bundles. 

 These structures consist in most cases of two parts, the 

 wood, which is the path for the ascent of water from 

 the roots, and the bast, which is more concerned with the 

 transport of the elaborated products of the metabolism of 

 the cells. 



The degree of development of this system varies very 

 much in different plants. In an ordinary herbaceous 

 Dicotyledon the bundles remain separate, and can be 

 traced separately from the root, through the stem to the 

 leaves (fig. 27) in which they form the branching network 

 known as the veins (fig. 28). With greater size, however, 

 more capacious channels are demanded, and we find more 

 and more bundles developed, until we reach the condition of 

 the oldest trees, nearly the whole of whose trunks are 

 formed of tissue which either is or has been devoted to 

 this service. In such trees the most actively living parts 

 are found at the extremities, by far the greatest number of 

 their protoplasts being situated in the twigs and leaves. 

 Indeed, the greater part of the wood of the trunk of many 

 trees is dead, and consequently functionless. 



The same tissues serve for transport in the Monocotyle- 

 dons, and in the Vascular Cryptogams, though the mode of 

 arrangement of the elements is altogether different from 

 that of the Dicotyledons. 



In those vascular plants which live in water, and 

 particularly in those which are totally submerged, there is 

 no need for so elaborate a transport system, as water can 

 be readily absorbed by the general surface. We find two 

 modifications of structure in such plants ; the epidermis is 



