PART IIL-PHYSIOLOGY. 



fp4 

 CHAPTER I. FUNCTIONS OF PLANTS IN GENERAL. 



82. It has been noted in Part II General 



that the essential living substance of plants consists of pro- Conditions ' 

 toplasm and that this protoplasm is capable of doing various 01 >lantLlfe - 

 kinds of work, of performing various functions, such as those of 

 nutrition, respiration, growth, movement and reproduction. See 

 page 68. 



Protoplasm can, however, only perform these functions 

 provided that the external conditions are favourable. If 

 no water is available protoplasm at once becomes inactive 

 and may die, while it is easily destroyed by too high or too 

 low a temperature. 



The protoplasm contained in the cell from the growth 

 and development of which a Teak tree is ultimately produced 

 must clearly have a very different structure from that contained 

 in the cell which eventually develops into a Sal tree : with the 

 bost microscopes at present available, however, no such 

 difference can be detected. Still we must not on that account 

 expect the protoplasm of different plants to always behave 

 in exactly the same way when the external conditions are 

 the same, and, as a matter of fact, we know that different 

 plants have different needs and that the effect of one and 

 the same factor on different plants may be very various. The 

 amount of light, for instance, which is absolutely essential 

 for the healthy development of some plants (ligkt-demanders) 

 is often injurious to those plants which live in shady places 

 (shade bearers). 



In all ordinary green plants, however, a suitable supply 

 of water and mineral salts, a suitable amount of light and heat, 

 as well as of free oxygen and carbon dioxide, are essential 

 for their existence and for the vital activity of their protoplasm. 



In Physiology, then, we must consider, on the one hand, 

 the living, irritable, protoplasm, and on the other hand, the 

 non-living factors of the plant's environment which are capable 

 of acting as stimuli to the protoplasm, of inducing it to behave 

 in a certain way and to perform definite work, while, under 

 certain circumstances, they may be able to altogether prevent 

 its activity and even to kill it. 



