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. Like other living beings plants sooner or 

 later perish. In spite, however, of the limited duration of life and 

 number of individuals, 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 most 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 inter- 

 course with the world outside. Consequently, the differently arranged 

 elements must be adapted for different modes of life, and, since they 

 exercise their functions in different ways, they 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 natural 

 groups of plants, there are certain differences as regards form 

 and function. In all plants, however, those organs to which the 



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