CH. Il] GAIN IN WEIGHT. 27 



10 a.m. to 5 or 6 p.m., when the amounts of starch are to 

 be compared. 



(36) Gain in Weight. 



Sachs^ 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 3 mm. in thickness cut 



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



piece of 10x5 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. Helianthus, Cucurbita, Rheum. The experiment 



must be begun soon after sunrise''. Having selected 5 or 



6 healthy leaves of, say Helianthus, each must be cut 



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 as many areas as possible consisting of 



mesophyll without 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. 



' Arbeiten, in. p. 19. 



^ 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. 



