GROWING PLANTS UNDER GLASSES OF VARIOUS COLOURS. 



60 



for Engelmann has shown that individuals of Bacterium tenno 

 only exhibit movements in the presence of free oxygen, arid 

 consequently congregate round a Conferva at two places, one 

 being between the lines b and c, the other to the right as well 

 as in the line f in the blue, that is, about the situations of the 

 two chlorophyllian absorption-bands No. I and No. V. It may 

 be also observed that the illustration shows the amount of 

 bacteria between b and c to be much in excess of that of the 

 organisms at p.* Similarly, MM. Bonnier and Mangin have 

 proved by a different method, viz. by comparing the respective 



values of the ratio S^- , that assimilation is discoverable both in 

 the violet and obscure ultra-violet rays, in addition to the 

 position in the red-orange part of the spectrum.t 



Under green glass vital activities of all kinds seem to be 

 greatly checked. Though the temperature is lowest of all, and 

 therefore the injurious effect of heat, so conspicuous under red 

 glass, is not so apparent, yet the percentage of water is the 

 next highest to that under red glass. This is probably due to 

 an opposite cause, namely, the feeble absorption-bands III and. 

 IV being only present ; and, therefore, the loss of water by 

 transpiration is much reduced. Moreover, the loss by respira- 

 tion not being increased by heat as under red, the percentage 

 of dry substance is nevertheless low, through the feeble assimi- 

 lative powers. 



Clear glass shows a slightly less percentage of dry substance 

 than yellow. This is doubtless due to the increase of tem- 

 perature ; though, by transmitting all rays, the assimilation and 

 transpiring powers are very active. 



Plants growing in the open air are in the best condition of 

 all. The temperature being nearly as low as under green glass, 

 the assimilative powers now rise to a maximum, and the per- 

 centage of dry substance is the greatest, that of water being, of 

 course, the least. 



* See the description and figure by Vines, op. cit. p. 250. 



t Bull, dc la Sue. Bot. dc Fr. 1885, torn, xxxii. 2. ser. 7, p. 3G8. 



