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 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 hag 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, 
