90 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



I selected six j^oung lettuce plants on May 15 and May 23 respec- 

 tively, from a bed, and transferred them to miniature pots. They 



were allowed to become well established, and then the pots were com- 

 pletely invested in gutta-percha sheeting as described. Each plant was 

 placed under the coloured glasses in succession for twenty-four hours, 

 with the following results (loss in grammes) : — 



Lettuces (first series). 

 Mean Temperature 64^. 



No. of Pot. 



Eed. 



Yellow. 



Green. 



Blue. 



Violet. 



Clear. 



I. . 



1-09 



1-16 



1-06 



1-29 



1^47 



1^29 



II. . 



•61 



•07 



•55 



•72 



•95 



•61 



III. . 



1-05 



1-16 



1-22 



1-25 



2^16 



•63 



IV. . 



1-43 



105 



1-16 



1^16 



1-92 



1-72 



V. . 



1-11 



1-09 



•89 



102 



1^48 



1^13 



VI. . 



1-39 



1-62 



1-48 



121 



1-75 



1-50 



Total 



6-68 



6-15 



6-36 



6^65 



9-73 



6-88 



The temperature during the six days occupied with this experiment 

 varied only eight degrees, from 56^ to 64*^ F. ; and to show how little 

 this affects the results, the temperature for pot I. under red glass 

 ranged from 59*^ to 64*^, while that for the same plant under yellow 

 glass was from 59° to 62° ; and yet in this instance the yellow glass 

 gave a rather higher loss than the red. iVgain, for No. II. the tem- 

 perature for red glass varied from 59° to 60°, that for yellow from 

 59° to 64°, showing that in both cases the temperature was lower, 

 when the loss was comparatively greater, contrary to what would be 

 expected, since it is known that an elevation of temperature is one of 

 the causes of increasing the loss of moisture. The differences, there- 

 fore, must be put down to the unascertainable variations in the amount 

 of sunlight. 



Hence will be seen the importance of a protracted series of experi- 

 ments ; and the results given under the line of "Totals" will un- 

 doubtedly be a much nearer approximation to the truth. 



These give one maximum for the red and another for blue and violet, 

 together; while yellow and green, singly or together, furnish decided 

 minima. 



An independent maximum is also supplied by clear glass. Com- 

 paring this with the effects of the yellow glass, which also transmits 

 red light, it seems that so far from the optically brightest rays being 

 a chief cause of transpiration, the yellow, and those rays on the more 

 refrangible side of it, where the chlorophyll absorption -bands, viz. 

 Nos. II., III., and IV., are feeblest, must have actuaUy a retarding 

 influence upon the effect of red and violet liglits, which per se are most 

 powerful. 



