58 



Influence of Solar Radiation on Plants. 



to the fixation of water, for there was no supply of carbonic 

 acid from the air, and the quantity of substance which the roots 

 could absorb from the bricks must have been very trifling. 



The comparative experiment in which the seeds were exposed 

 to the open air of the room did not come to anything. Not one 

 of the seeds succeeded even in bursting the tunic, doubtless 

 because the dry atmosphere prevented their ever retaining suffi- 

 cient moisture. 



The presence of soil about the germinating seeds, or a con- 

 stant change of air, would probably have modified these results ; 

 yet the conditions observed in this series of experiments were 

 thought necessary, in order to have the full effect of the different 

 sorts of light about the seeds themselves, for a soil necessarily 

 produces partial if not total obscurity. The deprivation of other 

 sources of carbon, beyond the cotyledons of the seed itself, also 

 answered certain purposes. On examining the results, the follow- 

 ing conclusions may be drawn, as far as wheat is concerned 

 growing under the conditions of the experiment. The absence 

 of the chemical rays favors the first growth of rootlets, and the 

 presence of the luminous rays does not impede it. Afterwards 

 the opposite effect takes place ; the roots are stopped in their 

 development by the yellow ray much more than by all the rays 

 of the spectrum in combination. The red or calorific ray is on 

 the whole the most favorable to the growth of the roots, even 

 more so than the complete absence of all solar radiations. The 

 shooting forth of the plume is also favored by the withdrawal of 

 the chemical rays, especially just at first ; but the full and healthy 

 development of leaves requires all the rays of the spectrum, the 

 luminous being particularly necessary. Several other peculiari- 

 ties may be noted ; for instance, the downward tendency of the 

 roots under the pure luminous influence; the comparatively 

 greater development and strength of the membranous sheath 

 under the calorific agency ; and the late but abundant growth of 

 side-rootlets, where all the solar radiations were admitted. 



The results of the experiments on wheat recorded in the pre- 

 vious Eeport, where there was the presence of soil and change 

 of air, appear to indicate still more clearly the beneficial charac- 

 ter of the luminous emanations, for the plants under the yellow 

 shade were found even to excel those which had grown in white 

 light, while, as in the experiment just detailed, the cutting off of 

 the luminous ray by the deep blue glass militated greatly against 

 the health of the plants. The protection of the rootlets from 

 the yellow ray may be fairly considered an advantage, but a 

 proper series of experiments on wheat-seeds surrounded by earth 

 is still a desideratum. 



We have now to consider the growth of the peas under the 

 different solar influences. It has been already stated that twelve 



