PART III. 



PHYSIOLOGY OF PLANTS. 



CHAPTER XI. 



THE CHEMICAL COMPOSITION OF PLANTS. 



I. After becoming acquainted with the external and internal 

 structure of plants, it is necessary to proceed to study the work 

 which the various parts perform in the maintenance of the life of 

 the plant : this branch of the science of Botany is termed physi- 

 ology. Among the higher forms of plants various members and 

 tissues are adapted to carry out certain functions or certain kinds 

 of physiological work ; the individual members and tissues by 

 which the functions are performed being termed organs of the 

 plant. 



It is at the outset important to emphasise the fact that all the 

 various functions are dependent upon the living protoplasm, and 

 that the activity and power of the latter to carry them on satis- 

 factorily is bound up with certain external conditions, namely, a 

 suitable temperature, adequate supply of food-materials, and in 

 the case of green plants a certain intensity of light, and access to 

 free oxygen of the atmosphere ; without the fulfilment of these 

 conditions death takes place and the various vital phenomena 

 cease. 



The functions of plants may be divided into two groups : — 

 (i) The nutritive functions which are concerned with the 

 absorption, elaboration, and appropriation of the food-supply 

 and therefore specially adapted to the maintenance of the life of 



the individual, 

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