PHOTOSYNTHESIS 



93 



The cutter works on the principle of a punch (Fig. 20). A stiff iron frame, 

 of a form conveniently grasped by one hand, carries steel dies operated by 

 pressure of the thumb. The dies cut discs cleanly from a leaf held between 

 them, the discs then dropping into a perforated aluminum cup screwed below 

 the lower die. The diameter of the punch-dies is as nearly as possible 1.128 

 cm., and hence every disc is 1 sq. cm. in area. In use the arms of the frame 

 are slipped above and below the leaf, which is guided by the left hand, when 

 the discs may be cut very conveniently and rapidly, in any desired number, 

 care being taken to avoid the larger veins. The cup is then unscrewed and 

 covered by its own screw cap, which projects sufficiently to allow the cup to 



Fig. 20. — Leaf-area cutter, with disc cups; X|. On the left is a cup 

 in a test-tube as arranged for steaming the discs. 



hang near the top of an ordinary test-tube, as shown in the figure. Water 

 in the bottom of the test-tube is then boiled in the gas-flame (any convenient 

 holder being used), and the steam enters the perforations of the cup and kills 

 the living cells of the discs, thus preventing any loss of weight by respiration. 

 The cup, with its contents, is then placed in the drying-oven. After a pre- 

 determined interval, using the other cup, an equal number of discs is cut 

 from the same leaves. These are treated as in the first series, and the cup 

 is placed beside the first in the drying-oven. When the discs are thoroughly 

 dry, which -requires two or three days, both cups are carefully weighed; then 

 the weights of the cups (stamped upon them, together with the letters M 

 and N respectively to distinguish morning and night cups) are subtracted, 

 when the remainder gives the dry weights. Both cutter and cups should 

 often be cleaned, to prevent rusting of the one and accumulation of dry sap 



