GROWING PLANTS UNDER GLASSES OF VARIOUS COLOURS. 75 



to 14*87 grs. (Table III. col. 1). In other words, the percentages 

 ( Table III. col. 4) show an increase of water and a decrease of dry 

 substance. The interpretation of this appears to be that tran- 

 spiration and assimilation were much too feeble and respiration 

 too active. 



The total calculated weight of dry substance by assimilation 

 under green glass (0*83 gr.) falls short of the calculated amount 

 of plants grown in the open by 7"80 grs. (Table II. col. G), the 

 percentage being only 5*3, as compared with 12-5. In every 

 way, therefore, green glass is proved to be most prejudicial in 

 arresting vital functions.* 



After green, red glass incites the least amount of assimi- 

 lative activity. Comparing the effects of these glasses with 

 those of yellow, the different influences of the heat and luminous 

 rays are well seen. Thus while the mean temperatures under 

 red and yellow are nearly the same, the total increase of weight 

 is far greater under yellow than under red, the gain being at 

 the rate of 263 per cent, for the latter and 435 per cent, for the 

 former. 



The effect of heat is perhaps best seen in the amount of water, 

 for the percentage is practically the same under both red and 

 yellow glasses (Table III. col. 4). 



When, however, we compare the results of assimilation, as 

 shown by the relative amounts of dry substance, then the 

 superiority of yellow light becomes very apparent. Thus, com- 

 paring the columns (1) and (6) in Table II., we see that the total 

 amount of dry substance acquired under yellow glass (3* 18 grs.) 

 is much greater than that under red (1'85 grs.) ; while the 

 deficiency from that obtained by plants grown in the open 

 (5-45 grs.) is less than that for red glass (6*78 grs.). 



* An account (together with Dr. Lindley's criticisms) of the preparation 

 of the pale emerald-green tinted glass of the Palm-house in Kew Gardens will 

 be found in the Gardeners' Chronicle, 1847, p. 524, and 1848, pp. 138, 155. 

 The main object was to reduce the scorching heat of the sun. The depth of 

 the colour is too feeble to be injurious ; but, as Dr. Lindley observed in his 

 Theory of Horticulture, p. 300, " No advantage seems to have resulted from 

 glazing the great Palm-house with green glass of a tint selected by Mr. 

 Hunt." On the other hand, the Fern-houses at Kew were glazed with a much 

 deeper tinted green glass. This, however, has now been removed, as it was 

 found to be injurious. (See art. in Gardeners' Chronicle, Nov. 26, 1892.) 

 The fact is, as more recent experiments show, that it is not only a certain 

 amount of heat which is cut off by green glass, but the more important 

 rays which are required for assimilative purposes. 



