62 



Influence of Solar Radiation on Plants. 



with chemical power, are the most suited to the production of a 

 healthy growth. The influences which facilitate rapid growth 

 are diametrically opposed to healthy development. It should be 

 borne in mind, however, that these observations relate only to a 

 very early stage of the plant, and teach us nothing respecting the 

 full-grown pea. or the evolution of the flower or fructification. 



If we compare the effect of the various solar radiations upon 

 the germination of wheat with the effect produced upon that of 

 peas, we are struck with the great diversity between them. This 

 was particularly apparent during the progress of the experiment. 

 The colorless and the red glasses happened to stand side by side 

 on the table, and it was curious to notice under the former glass 

 a tall and vigorous crop of corn-plants with a mere matting of 

 stunted roots from the peas, while under the other a thick crop 

 of green spreading plants arose from the germinating peas, but 

 the wheat-plants were few, straggling, and unhealthy in appear- 

 ance. When, however, we come to look more closely into the 

 phenomena, we see certain points of resemblance. In both 

 cases the cutting off of the chemical ray facilitates in a marked 

 manner the process of germination, and that both in reference to 

 the protrusion of the radicles and the evolution of the plume. 

 The unnaturally tall growth of the stem, and the poor develop- 

 ment of leaves in darkness, more or less complete, is also common 

 to both these specimens of the monocotyledonous and dicotyle- 

 donous plant. In both cases too, the yellow ray exerted a re- 

 pellant influence upon the roots, giving the wheat a downward 

 and the pea roots a lateral impulse. 



The object of employing a partially obscured yellow glass in 

 these experiments, was to decide if possible the question which 

 has been asked, Does yellow light stop germination by some 

 specific action or merely by the excess of light ? Contrary to 

 the experience of some others, who, I believe, have experimented 

 on seeds covered with soil, and on other plants than those em- 

 ployed by me, the yellow light did not interfere at all with germ- 

 ination, in the experiments just described. In the case of both 

 plants, indeed, it decidedly facilitated the early development of 

 both the root and the plume. That the yellow ray, however, 

 has a specific action of its own, is proved by the most cursory 

 glance at the facts already recorded ; the yellow and the obscured 

 yellow give quite different results from those of any of the other 

 glasses. 



The diversity between the effect of the same qualities of light 

 upon the growth of the wheat and the pea, leads us to look with 

 suspicion on any generalisations affecting other plants which 

 may be drawn from the observed influence of light upon one 

 particular plant, especially, of course, when they are of different 

 orders. This will account for some of the diversity in the state- 

 ments made by previous experimenters in these fields, 



