i 3 8 BOTANY OF THE LIVING PLANT 



imitation is practically limited to the green plant. As men- 

 tioned in the Introduction the activities of the plant in this way are 

 of fundamental importance to animals, as well as to non-green plants ; 

 such organisms being dependent for their food on the organic materials 

 lynthesised by the green plant. Animals can utilise organic food 

 only, and the food of an animal is always derived, directly or in- 

 directly, from plants. An herbivorous animal feeds directly on plant 

 materia] While a carnivorous animal feeds on other animals, 

 the latter will probably be herbivorous in diet. And thus the so-called 

 "food-chains" are established which always terminate in the green 

 plant. This holds not only on land but also in the sea, where minute 

 plants such as the Diatoms are the ultimate source of the food of 

 all marine animal organisms. 



In animals food is used partly for purposes of body-building, in 

 which connection the Vitamins, essentially of plant origin, are of 

 recently-discovered importance ; also as a source of energy for vital 

 activities. The energy which an animal obtains by respiratory oxida- 

 tion of food materials is derived from the sun, and was originally 

 fixed by some plant, in the process of photosynthesis. Thus the whole 

 organic world depends on the sun for the energy which its vital 

 processes require ; all this energy is originally trapped by green 

 plants in photosynthesis. The future may," however, see some reduc- 

 tion in the dependence of the animal kingdom on plants ; for means 

 may be devised for synthesising food materials without the co-opera- 

 tion of the plant. 



In addition to the energy which we consume physiologically, the 

 vast supply of energy which modern civilisation requires is largely 

 vegetable in origin. Materials such as wood, coal, peat and prob- 

 ably petroleum have been ultimately derived from plants, and 

 the energy liberated from their combustion was originally fixed 

 in photosynthesis. Thus we return into present currency balances 

 of the sun's energy stored in the Earth's crust from an earlier 

 age. 



These general remarks are, however, a digression from the study 

 of the plant. Returning to the green plant itself, its constructive 

 metabolism provides new material. This is required in the first 

 instance for the nourishment and growth of its several parts : that 

 is for the maintenance of the individual. But secondly, it is required 

 for the increase in number of individuals. The propagation of the 

 race can only be carried out when sufficient material is at hand 

 from which to form new germs. 



