170 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



the increase in weight due to the accumulated products of 

 photosynthesis formed during exposure to light. 



A better method is to compare pieces of the same leaves, 

 using plants with large leaves, e.g. Sunflower, Tobacco 

 Plant. Select a number of symmetrical leaves the halves 

 on either side of the midrib approximately equal and divide 

 each in two longitudinally by cutting with scissors close to 

 the midrib. Find the area of the removed half -leaves by 

 cutting out a paper model of each half-leaf and weighing 

 these paper models against measured rectangular pieces of 

 similar paper, until by balancing you get the total area of 

 the half -leaves. Kill the leaves by steam, dry them, and 

 record their dry weight. Expose the plant to light for a 

 few hours ; then remove the remaining halves of the same 

 leaves (cutting along by the midrib) and kill, dry, and weigh 

 them. Reduce the resulting weight-increase to grams per 

 square metre per hour. 



225. Cranongf's " Leaf-area Cutter." Fig. 39 shows an ex- 

 tremely useful instrument invented by Prof. Ganong and supplied 

 by the Bausch and Lomb Optical Company, for rapid and accurate 

 cutting-out of discs \ sq. cm. in area from leaves. An iron frame, 



which can be held in one hand, 

 carries steel dies operated by 

 pressure of the thumb, the dies 

 cutting discs from a leaf held 

 between them and the discs 

 then falling into the perforated 

 aluminium cup attached below 

 the lower die. 



The arms of the frame are 

 slipped above and below the 

 leaf, which is guided by the 

 other hand ; any desired num- 

 ber of discs may be cut from 

 the leaf, care being taken to 

 avoid the larger veins and the 

 discs being cut alternately from 

 the two sides of the midrib ; 

 the cup containing the discs is 

 then unscrewed and covered by its own cap, so that the cup will 

 hang into a test-tube which is partly filled with water and heated 

 over a flame ; the steam enters the perforations of the cup and kills 

 the leaf discs ; the cup with its contents is then placed in the 

 drying-oven. 



Fig. 39. Ganong Leaf Area Cutter. 

 (Bausch and Lomb Optical Com- 

 pany.) 



