CHAPTER I. — GP:NERAL PHYSIOLOGY. 671 



generally spores, each of which is capable of developing into a new 

 organism (see pp. 3, 67). 



§ 3. The External Conditions. The functions of the plant 

 can only be carried on under a certain coincidence of favourable 

 external conditions. Thus, an ordinary green plant will only 

 flourish when the conditions are such that it is supplied with 

 appi-opriate food, with water, and with oxygen for its respiration, 

 and is exposed to a suitable temperature and to sufficiently in- 

 tense light. 



The importance of a supply of food and of water is sufficiently 

 obvious to need no further explanation here. The importance of 

 a supply of oxygen is that without it the normal catabolic pro- 

 cesses which are attended by an evolution of kinetic energy in 

 the plant would either cease, or be so far suppressed that the plant 

 would no longer manifest its vital phenomena ; for instance, it 

 would cease to grow, and would eventually die. But it must not 

 be assumed that these catabolic processes are the ultimate source 

 of energy in the plant : on the contrary, the energy evolved in 

 the decomposition of a substance only represents a part or the 

 whole, according to the degree to which decomposition proceeds, 

 of the energy expended in producing the substance. And here it 

 is that the importance of heat and light to the plant becomes 

 apparent; for the heat and light of the sun are the ultimate 

 sources from which the plant obtains the energy necessary for the 

 maintenance of its life. Heat is an essential condition to the per- 

 formance of all the vital functions of the plant : and Hght is an 

 essential factor in that most fundamental of all the nutritive 

 processes of the green plant, the assimilation of carbon dioxide. 



Inasmuch as the influence of heat and light is so comprehen- 

 sive, it may be generally considered now, the detailed considera- 

 tion of these and other external conditions being relegated to the 

 discussion of the functions which they especially affect. 



Heat. Every function of the plant can only take place within 

 certain limits of temperature : that is, between a certain minimum 

 and a certain maximum degree. Between these limits there is for 

 each function a degree of temperature, the optimum, at which that 

 function is carried on with the greatest activity ; any fall of 

 temperature from this optimum, or any rise above it, leads to a 

 diminished activity of the function. These general laws have been 

 arrived at by observation of such processes as movement, absorp- 

 tion by the roots, assimilation, etc. 



v. S. ]',. X X 



