THE DIFFERENTIATION OF THE PLANT-BODY 35 
solid core of wood, is comparatively little affected by the 
flexures of the structure, and its function is not interfered 
with. 
Another kind of differentiation in such a cell-mass as 
we are dealing with, is the setting apart of particular 
groups of cells for various metabolic 
purposes. We have the formation of 
glandular tissue, of the laticiferous 
systems, and so on. This differentia- 
tion may be marked also by the pro- 
duction of definite organs in the 
protoplasts, such as are seen for 
instance in the case of the chloro- 
plasts of the leaves (fig. 33) and 4, 955 curoroptasea 
other green parts of plants. EMBEDDED IN THE Pro- 
The habit of life of a plant again — TPA5M OF A Cin oF 
may influence its structure and the ov a Lear. 
degree of differentiation of its body 
to a very great extent. The great group of the Fungi afford 
us an illustration of the degradation of structure which ac- 
companies a saprophytic or parasitic habit. Similar instances 
of degradation are met with among the flowering plants. 
The needs of the cell-mass thus usually lead to the 
differentiation in its substance of at least four physiologi- 
cally different regions—the tegumentary, the conducting, 
the supporting, and the metabolic. The latter includes all 
the parts in which the protoplasts are comparatively little 
changed, and consequently are most concerned in carrying 
out the vital processes. 
The needs of the protoplasts forming the community of 
the plant include, however, as we have seen, something 
more than the arrangements so far described serve to secure 
for them. Each protoplast must be furnished with a certain 
amount of air, or rather oxygen. Almost all living sub- 
stances must carry on during life the process known as 
respiration. The free-swimming zoospore to which we 
have so often referred obtains a supply of oxygen from the 
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