$6 THE FACTORS [Part I 



changes, and a maximum, a minimum, and a mean value of L are dis- 

 tinguished from one another. 



If, for instance, it is stated that a plant thrives when L is i/i-i - 1/7, we should then 

 understand that it will grow under daylight of almost full intensity and also under 

 light one-seventh as strong, but not under less. L (max.) = J denotes that, at 

 a certain time of day, the light in the crown of a tree rises up to i of the full day- 

 light (I) ; L (min.) = J^, on the other hand, indicates that at a certain time of day it 

 sinks down to ^ of I. 



Wiesner's methods, of which the above is a short sketch, and of which 

 a full account is given in his cited works, ought in course of time to be 

 completed and, if possible, extended to the less refrangible rays. As 

 far as they go at present, they already form an indispensable aid to 

 physiological research that relates to phyto-geography. 



3. PLANT-LIFE IN DARKNESS. 



As has been already shown, there is nowhere on earth a place too 

 cold for plant- life, and only a few spots of very limited area that are too hot. 

 As regards light, there is no limitation ; it is nowhere too dark, nowhere 

 too bright to exclude plant-life of some kind. In the depths of ocean, 

 where light is completely absent, the decaying corpses of animals are 

 decomposed by bacteria. The dung of cavicolous animals becomes mouldy ; 

 the shaggiest skin, the thickest hide does not protect an animal's body 

 from the attacks of pathogenic plant-parasites. Vegetation in the dark 

 is, however, limited to plants that are nourished at the expense of organic 

 matter. The reduction of carbon from carbon dioxide by the chlorophyll- 

 containing organs is an operation due to light. Organisms that derive 

 the carbon they require from carbon dioxide thrive in the dark, so long 

 as the organic reserve material suffices, and then perish from want of food. 



The reduction of carbon dioxide is not the sole function induced by 

 light in the plant-organisms ; on the contrary, the same source of energy 

 is used for numerous other operations. Thus the formation of chlorophyll, 

 except in cryptogams and gymnosperms, demands the presence of light ; 

 the same is true in reference to other pigments, especially red and blue ones. 

 The assimilation of nitrates in the higher plants is strongly promoted by 

 light. Foliage-leaves remain very small in the dark. Many movements 

 of plants are excited only by light, others again are arrested by it. 



Shoots developed in darkness differ in many respects from normal 

 shoots and are said to be blanched or etiolated. They are devoid of chloro- 

 phyll, and therefore are white or yellowish. Their axes are much longer 

 than they are under normal circumstances ; their leaves, on the contrary— 

 except those of grasses and of a few other Monocotyledons — are very 

 small and generally deformed. Flowers are only rarely produced, even 



