SRCT. II 



PHYSIOLOGY 



179 



a cliemical reaction in which a particular substance S is formed may be accelerated 

 by temperature as is represented in the curve AB ; if, however, the substance S 

 is at the same time decomposed by the rising temperature, its actual amount may 

 be represented by the curve CD ; the effect of the rise of temperature on the 

 chemical reaction is expressed by the curve AE, wliich is the resultant of CD 

 and AB.(2) 



The cardinal points of temperature are usually on the average much lower for 

 the plants of cokl climates than for those of temperate or tropical regions. Thus 

 the geographical distribution of plants is in the first instance dependent on these 

 cardinal points. The highest cardinal points are found not in tropical plants, 

 but in certain Bacteria which can live in fermenting substances at a temperature of 

 70° C. 



At low temperatures in the neighbourhood of the freezing-point of water many 

 plants are killed by being frozen. This 

 may happen to susceptible plants at a 

 temperature above 0° C. and long before 

 ice is formed in tlie tissues. Other 

 plants can endure the formation of ice 

 within these, and may be thawed from 

 a hard, frozen condition still alive. The 

 resistance of lower organisms to extreme 

 cold is noteworthy. In Pictet's experi- 

 ments Diatoms endured for a long time 

 a temperature of - 200° C. ; various 

 Bacteria, according to the investigations 

 of Macfadyen and others, can support 

 a six months' exposure to 200° and even 

 to 250° of cold, produced by means of 

 liquid air and liquid hydrogen (•^). 



The demands of plants on light also 

 show much variety. Some require full 

 sunlight in order to thrive successfully, while others prefer the shade of woods or 

 of caves or clefts in rocks (shade-plants). 



By increase of the intensity of light any cell can be killed ; in different cases 

 the action of the light may be either mainly chemical or mainly thermal. Many 

 Bacteria are killed even by bright daylight ; on this depends the important 

 hygienic effect of light in houses and dwelling-rooms. 



The need of light not only changes from one species of plant to another, or from 

 individual to individual, but the optimum effect of light may change for the same 

 individual as it develops. Many of the cultivated plants of the tropics, e.g. 

 Colfee and Cocoa, require shade when young, and require to be at first protected by 

 shade-giving trees (species of Albizzia) planted for this purpose. When older they 

 bear or even require exposure to the full tropical sun. 



In addition to the energy which the plant obtains from the rays of light and heat, 

 numerous influences of substances in the plant's environment have to be considered. 

 This is not the place to enumerate the elements necessary to plants, but the need 

 of water, which is familiar to every one, may be mentioned. 



Fig. 175.— Explanation in Text. 



