THE DIFFEKENTIATION 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 FIG 33 ._^ LOKOPLA!m 



Other green parts of plants. EMBEDDED ix THE PKO- 



The habit of life of a plant again Z"p C Ss 

 may influence its structure and the OF A LEAF. 

 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 



3* 



