36 VEGETABLE PHYSIOLOGY 



CHAPTEK 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 dis- 

 positions 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 condi- 

 tion 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, however, 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- 



