193 BOTANY. 



been shown (1) that the assimilation is greater in the whole 

 bettm (white light) tlian in any one of its constituent rays, 

 and (2) that the amount of assimilation varies greatly in the 

 diflerent rays.* "When plants are grown in the different 

 rays of the spectrum, and properly protected, so that each 

 receives but one kind of light, the amount of assimilation in 

 Hich case is about as follows, that for white light being 100 : 



The less refrangible rays are thus seen to be far more effica- 

 cious than the more refrangible ones, and in the yellow and 

 orange rays, which are the biightest to the eye, the greatest 

 amount of assimilation takes place. From these rays there 

 is a decrease toward each end of the visible spectrum, and in 

 the so-called heat rays and chemical rays, found respectively 

 beyond the red on the one hand and the violet on the other, 

 there is no assimilation whatever. 



252.— Light and Metastasis. Many of the metastatic 

 changes in the plant take place in complete darkness, such 

 as those connected with the growth of roots and other sub- 

 terranean organs. In trees and thick-barked shrubs the metas- 

 tatic changes which occur in the stems are in total darkness, 

 and even in many herbs the thick cortical tissues must cut 

 off the greater part of the light from the active interior cells. 

 On the other hand, in a great number of aquatic plants their 

 translucency is so great that every internal change must be 

 in bright light, and in a few terrestrial plants — as, for ex- 

 ample, in Impatiens Balsamina — the cortical tissues permit 

 most of the light to penetrate to the inner active cells. These 

 facts indicate a marked indifference of the metastatic changes 

 to light, as compared with those of assimilation. 



This indifference is further illustrated in the growth of 

 flowers in the dark, where, with few exceptions, they develop 

 as perfectly as in the light. So the colorless parasites — e.g., 

 Monotropa, Aphyllon, Corallorhiza, etc. — and all the fungi 



* The earliest experiments of much value were those of Charles 

 Daubeny, " On the Action of Light upon Plants, and of Plants upon 

 the Atmosphere," pub. in PhU. IVans., 1836. 



