CH. II] GAIN IN WEIGHT. 31 



(36) Gain in iveight. 



Sachs l has shown that a given area of leaf is heavier 

 in the evening than in the morning, owing to the accumu- 

 lated products of assimilation. 



The following are Sachs' instructions for performing 

 the experiment. Out of a board 3mm. in thickness cut 

 out a square of 10 cm. to the side and another rectangular 

 piece of 10 x 5 cm. : these are to be used as templates by 

 which to cut out areas of 100 sq. cm. and 50 sq. cm. 

 respectively. The plants used must be large leaved kinds, 

 e.g. Heliantlius, Cucurbita, Rheum. The experiment must 

 be begun soon after sunrise 2 . Five or six healthy leaves 

 having been selected, each must be divided longitudinally 

 close to one side of the midrib ; the part which is 

 thus freed from the plant is to be investigated at 

 once, while the other half remains on the plant till the 

 evening. Each half- leaf is treated in the following way. 

 It is laid on a flat board, the lower side of the leaf being 

 upwards, so that the projecting veins may be easily seen. 

 The templates are now fitted in between the larger veins 

 so as to get areas as free as possible from large veins. 

 The rectangular pieces of leaf so obtained are quickly 

 killed by steam. After being allowed to become air dry, 

 they are powdered, dried, and weighed. 



In the evening a similar process is gone through with 

 the control halves. The following is the result of one of 

 Sachs' experiments. A hundred sq. cm. were cut out of 



1 Arbeiten, in. p. 19. 



2 Unless the plant is placed in a dark room on the previous evening, 

 in which case the operator chooses his own time in the morning. 



