26 VEGETABLE PHYSIOLOGY 



conducting system. A little reflection will show us the neces- 

 sity for the development of some such system as this, which 

 must be more extensive and complex as the size of the 

 plant increases. We find that the source of water on which 

 a terrestrial plant depends is the soil in which its roots are 

 embedded. Even when it is young many of its protoplasts 

 are placed at a considerable distance from such a source of 

 supply, and in the absence of a ready means of communica- 

 tion 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 certain salts dissolved in it. 

 Much of this water must be evaporated to enable continuous 

 absorption to take place. 



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 last, 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. 



