LIGHT AND VEGETATION. 199 



stature, two of reduced size, while the remaining two take the form 

 of club-shaped bracts. 



The diminished size of the perianth of cleistogamous flowers of 

 such types as the violet is due directly to the action of diminished 

 light upon the hidden or inclosed flower. 



The influence of light upon the structure, reproductive processes, 

 and distribution of the lower forms brings about the most widely 

 divergent reactions, which can not be described here. 



The distribution and color of marine algae depend upon the depth 

 of the water and the consequent intensity of the light. This gives 

 rise to distinct zones of aquatic vegetation. Thus in one series of 

 surveys the littoral zone, the beach area covered at high water and 

 exposed at low water, was found to furnish proper conditions for 

 green, brown, and red algae. The sublittoral zone, extending to a 

 depth of forty metres, is furnished with red algae, increasing in num- 

 ber with the depth, and the brown algae disappear; while the elittoral 

 zone, from forty to one hundred and ten metres, is inhabited by red 

 algae alone. The number of species of vegetal organisms below 

 this depth is extremely small. An alga (Halosphceria viridis) 

 has been brought up from depths of one thousand to two thousand 

 metres. 



A very great number of bacteria are unfavorably affected by 

 light, and find proper conditions at some depth in the soil or water. 

 It is on account of this fact that the water of frozen streams becomes 

 more thickly inhabited by certain organisms than in the summer 

 time, and exposure to sunlight is adopted as a hygienic measure in 

 freeing clothing and household effects from infection. Bacteria 

 occur abundantly in sea water at depths of two hundred to four 

 hundred metres, and quite a number of species are to be found at 

 eight hundred to eleven hundred metres. 



The distribution of fungi follows the general habit of bacteria 

 in that they thrive best in darkness. 



It is to be noticed in this connection that light is also a deter- 

 mining factor in the distribution of the higher land plants. Thus 

 the amount of light received in polar latitudes is quite insuffi- 

 cient for the needs of many species, entirely irrespective of tem- 

 perature. 



The retarding influence of light upon growth is even more 

 marked in the lower forms than in the higher. Such action is the 

 result of the disintegrating effect of the blue-violet rays upon fer- 

 ments and nitrogenous plastic substances. 



The greater massiveness of the bodies of the higher plants en- 

 ables them to carry on the chemical activities in which these sub- 

 stances are concerned in the interior, where the intense rays may not 



