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 
