DIV. ii PHYSIOLOGY 219 



plants) ; others can pass through their whole life-history in complete 

 darkness. 



The necessary factors must further be present within certain 

 definite limits. An excess (above the maximum) or too little (below 

 the minimum) is alike injurious, and at a certain intensity (optimum) 

 the best results are obtained. MINIMUM, OPTIMUM, and MAXIMUM are 

 recognisable in the dependence of every vital phenomenon on an 

 external factor, and are called the CARDINAL POINTS of the influence 

 of this factor. They are by no means constants ; they differ for 

 particular organisms and particular vital phenomena ; they change 

 with the duration of the influence of the factor, and they depend on 

 the condition of the plant, and on other external factors. 



Every transgression of the minimum, or the maximum, for an 

 external factor leads sooner or later to death. This may result from 

 too high a temperature or from too low a temperature, from too 

 much or too little light, or from an excess or an insufficiency of 

 some substance. Thus when too little water is given a plant dries 

 up, or when a substance is present in excessive and injurious amount 

 a plant may be poisoned. 



Most plants are killed by being frozen ( 3 ) at sufficiently low temperatures. 

 Nearly all are killed by high temperatures that are far below the boiling point 

 of water. Only some Cyanophyceae can endure the very high temperature of 

 certain hot springs. 



Susceptible plants, especially those of a tropical climate, are killed even at 

 temperatures above C. Others are killed by the formation of ice in the tissues, 

 while some may be frozen hard in winter without suffering any harm. Cochlearia 

 fenestrata in Northern Siberia endures a temperature of - 46 C. without injury, 

 and some forest trees can stand even - 60 C. The resistance of lower organisms 

 to extreme cold is noteworthy. Thus in PICTET'S experiments Diatoms endured for 

 a long time a temperature of - 200 C. 



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. 

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

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

 they bear or even require exposure to the full tropical sun. 



Among the influences of particular substances that of WATER is especially 

 evident. When light and temperature are at the optimum, as is the case in the 

 tropics, the development of plants depends above all on the supply of water. In 

 regions with a large rainfall, uniformly distributed throughout the year, a most 

 luxuriant vegetable growth occurs as in the formation of the TROPICAL RAIN 

 FOREST. A regularly recurrent dry period determines DECIDUOUS FOREST, a lesser 

 rainfall permits of the formation of SAVANNAHS, and still more reduced precipita- 

 tion leads finally to a DESERT ( 4 ). %. 



Few plants can bear prolonged drying and the associated loss of water. Often 



