36 VEGETABLE PHYSIOLOGY 
CHAPTER III 
THE SKELETON OF THE PLANT 
In the last chapter we discussed the differentiation of the 
body of the plant, and examined the constitution of various 
mechanisms which are associated with such differentia- 
tion. If we study the arrangements which are peculiar to 
any plant, we shall find that almost all such differentiation 
as exists involves a modification of the non-living part, and 
particularly the walls of the supporting and conducting 
tissues, the living protoplasts having fundamentally the 
same structure or composition, whatever may be the nature 
of their immediate support. All the various dispositions 
of the non-living elements or structures are secondary in 
importance to the protoplasts. 
We cannot, indeed, lay too great stress on the fact that 
the needs and conditions of the protoplasts are primarily 
the causes of the differentiation of the non-living structural 
parts, and such differentiation is the expression of the fact 
that division of labour has arisen among the protoplasts of 
the community. 
' We have seen that a protoplast in its simplest con- 
dition is capable of an independent existence without any 
form of mechanical support beyond that which it derives 
from the slight difference of density between its external 
layer and its interior. In most cases, howeyer, this is 
not sufficient for protection during its whole life, and a 
membrane is subsequently formed around it. The mem- 
brane itself is a secretion from the protoplast, which in 
fact prepares its own defensive mechanism. In most cases 
the protoplast is always clothed by a cell-wall, the forma- 
