ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 59 



the expedients which Lindley suggests. The only alternative is 

 to do what we do at Kew — lower the temperature as much as 

 possible, and so secure the highest possible relative humidity, 

 with the double result of keeping the plants at rest and of check- 

 ing their desiccation." 



I hope soon to prepare an illustrated and detailed monograph 

 in which I shall be able to justify many of the statements which 

 occur in the body of this report. 



ON SOME EFFECTS OF GROWING PLANTS UNDER, 

 GLASSES OF VARIOUS COLOURS. 



By the Rev. G. Henslow, M.A., F.L.S., &c. 



[Read March 14, 1893.] 

 Introduction. 



Of the three important functions executed by living protoplasm 

 under the influences of solar radiations — namely, respiration, or 

 the inspiration of oxygen and the expiration of carbon-dioxide ; 

 transpiration, or the exhalation of watery vapour; and assimi- 

 lation, or the inspiration of carbon-dioxide and the expiration 

 of oxygen — it has been clearly established that the heat-rays are 

 the prime stimulators of the first, or respiration, which may, 

 therefore, be independent of the luminous rays. Under ordinary 

 circumstances there is an optimum temperature for each kind of 

 plant ; and if it be continually raised artificially, respiration also* 

 continues to increase, indicated by the ever-increasing quantity 

 of carbon-dioxide expired, until death ensues ; the curves 

 representing the increments of carbon-dioxide disengaged corre- 

 sponding to the increments of temperature being in all cases 

 parabolas. 



Experiments are wanting to ascertain under which colour 

 or colours respiration attains maxima ; but as the curve of 

 obscure heat in the solar spectrum has one maximum between 

 the lines B and C, i.e. corresponding pretty accurately with the 

 chlorophyll absorption-band No. I., we may assume with some 



