88 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Clear. — Ordinary transparent colourless glass, of a thickness of one- 

 tenth of an inch. 



Nature of Experiments. — In commencing my experiments I at 

 first employed cut shoots and detached leaves of various plants, as has 

 so generally been done by previous experimenters. In every case they 

 were weighed both before and after their exposure for definite periods 

 to daylight transmitted through the coloured glasses. Different 

 methods were adopted ; but the most satisfactory for short periods, 

 such as are indeed only possible with detached specimens, was to cut 

 the shoot under distilled water, inserting it in a small test-tube sub- 

 merged at the same time. This secured the cut end from exposure to 

 the air. The surface of the water in the tube was prevented from 

 evaporating by a few drops of oil forming a thin layer. The whole is 

 easily and accurately weighed to the 500th part of a gramme. The tube 

 with the shoot or leaf thus prepared was enclosed in a box covered 

 above with the sheet of coloured glass. 



Although experimenters have so generally employed cut shoots and 

 leaves, I soon found, on making my calculations from data accumulated 

 from the weights, that although some may appear quite fresh to the 

 eye for three or even more days after the commencement of the experi- 

 ment, yet the vitality of the shoot or leaf had, nevertheless, been 

 becoming enfeebled all the while, and the amount of water transpired 

 steadily decreased day by day irrespective of the characters of the 

 coloured glasses used; so that finally the relative amount of loss in 

 successive days became untrustworthy. 



To ascertain the differences in the amount of water transpired 

 under the influence of different rays of the spectrum, prolonged and 

 repeated exposure of the same specimen to the same kind of light is 

 necessary. The reason for this is obvious; for there are (as stated 

 above) so many disturbing influences which may materially affect the 

 results, that unless they be reduced to a minimum the effect due to 

 colour alone cannot be even approximately ascertained. 



If, however, the experiments be carried on under conditions which 

 will reduce the above disturbing elements to a minimum, then the 

 differences due to the colour of the light transmitted will be the most 

 powerful agent in the process of transpiration ; and it is only by taking 

 the mean of many experiments that the above influences can be virtually 

 eliminated. 



In attempting to do this with cut specimens, none will be found to 

 last long enough unaffected by the lesion to give very trustworthy 

 results. Hence, although such experiments may tend to corroborate 

 those obtained by more perfect methods, yet I do not think it worth 

 while to enumerate more than one or two of my own, numerous as 

 they have been, as by themselves they would not furnish a sufficiently 

 accurate basis for induction. They may have their use, however, in 

 sliowing the somewhat negative, or at least uncertain, results which 

 are generally only obtainable from cut specimens. 



