30 ; VEGETABLE PHYSIOLOGY 
lateral bending, the supporting tissue is situated near the 
periphery of the stem, and the latter is often still further 
straightened by being furnished with ridges or flanges. 
An instance of an almost converse character is afforded by 
a young root. In its growth, while it must possess sufficient 
rigidity to enable it to penetrate the soil, it must be capable 
of frequent bending to enable it to avoid obstacles. This 
is most advantageously provided for by a central core of 
strong tissue, surrounded by more succulent material. 
The transporting tissue of the centre is comparatively little 
affected by the flexures of the structure, and its function 
is not interfered with. This arrangement serves also as a 
protection against uprooting. 
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 proto- 
plasts, such as are seen for instance in 
the case of the chloroplasts of the leaves 
(fig. 833) and other green parts of plants. 
Fic. 33.—CHLoRopLasts The habit of life of a plant again 
EMBEDDED IN THE Pro- . 
ropLasm or a Cett or May influence its structure and the 
Or a Laue tpe TISSUE 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 accompanies 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 
