SECTION II 



PHYSIOLOGY 



Plants, like animals, are living organisms. Beginning their develop- 

 ment with the simplest structure, and increasing in size from internal 

 causes, they assume their definite form and complete their existence 

 according to laws determined by inheritance. Surrounded by a world 

 which differs very widely from them as regards chemical constitution, 

 they produce the substances necessary to their growth from the raw 

 materials afforded by the environment. To this end the different parts 

 of their bodies are enabled by independent movements to take such 

 relative positions as are most favourable to their mode of growth. In 

 spite of the number of individuals and the limited duration of life, the 

 continuance and extension of the species are provided for by an ability 

 to reproduce like organisms. 



Nourishment, independent growth, power of movement, and 

 reproduction are, together with respiration, the striking attributes 

 which characterise plants as living organisms, and distinguish them 

 from all lifeless bodies. 



An organism consisting of but one cell, as is shown by the life of 

 the simplest plants, is capable of exercising all the functions necessary 

 for the continuance of its existence. In the case of plants, however, 

 which consist of many hundreds or thousands of cells arranged in three 

 dimensions of space, it is impossible, for purely physical reasons, that 

 all the cells should bear the same relations to the outer world. The 

 cells in the interior must exist under conditions altogether different 

 from those which are in direct contact and intercourse with the world 

 outside. Consequently, the differently arranged elements must be 

 adapted for different modes of life, and, since they must exercise their 

 functions in different ways, must show what is called differentiation. 



This necessary division of labour has led to the development of 

 external organs and internal structures wonderfully adapted to the 

 requirements of the whole plant. Correlated with the various Classes 

 and relationship of plants, there are certain differences as regards 



