H4 METABOLISM 



siderably deepened if we proceed to investigate the subject quantitatively as 

 well as qualitatively. Quantitative researches were first carried out by SACHS 

 (1884), who was able, by using the iodine method, to obtain an approximate 

 estimate of the amount of the assimilation products. His procedure was as 

 follows : He selected large, well-developed foliage leaves on plants growing 

 in the open (Helianthus, Cucurbita, Rheum), and early in the morning cut out 

 of one longitudinal half of the leaf (avoiding the larger veins) accurately measured 

 portions from 50 to 100 sq. cm. area, in all about 500 sq. cm., and determined 

 their dry weight. In the evening corresponding areas were taken from the 

 other longitudinal half, which meanwhile remained attached to the plant and 

 had been assimilating, and these were treated in the same way. In every case 

 a marked increase in dry weight was observable, equivalent to the following 

 amounts in grams per square metre per hour : 



Helianthus, 0-914 g ; Cucurbita, 0-680 g. ; Rheum, 0-652 g. 



These numbers, however, must, for two reasons, be considered as representing 

 only a fraction of the products actually manufactured. In the first place we 

 know that every part of the plant suffers considerable loss of organic substance 

 during carbon-dioxide assimilation, owing to respiration acting in the reverse 

 direction (Lecture XVI), and, further, that there is a continual withdrawal of 

 soluble carbohydrate taking place (Lecture XIV) into the stem. SACHS did not cal- 

 culate exactly the loss from respiration, and disregarded it, beyond estimating 

 it at about i g. per sq. metre during fifteen hours of assimilative activity. On 

 the other hand, he instituted special experiments to estimate the amount of the 

 translocated materials. With this object he investigated the loss in weight taking 

 place during ten hours of continuous darkness, employing the same ' half-leaf ' 

 method above described. The temperature is, however, higher by day, so that 

 more material is removed by day than by night. Since SACHS did not assume 

 a greater loss in weight during assimilation than at night, his numbers may be 

 taken, at all events, as not too great. He attempted to determine in another 

 way the loss due to translocation, viz. by using isolated leaves ; in this case, 

 also, the numbers obtained were certainly too small because storage of the 

 products of assimilation must inhibit carbon-dioxide decomposition (Lec- 

 ture X). The following numbers may be taken as giving only approximate 

 minimum values under very favourable external conditions : 



Helianthus per hour and sq. m. (Method i) 1-882 g. of dry weight formed 



v V. 2 ) r> 7 



Cucurbita ( i) 1.502 



Taking these numbers as a basis, SACHS reckoned that a vigorously active 

 sunflower can manufacture 36 g., and a cucumber, 185 g. of dry weight 

 during the course of a warm, bright summer day. 



In SACHS'S memoirs it is not the dry weight that is spoken of, but always starch, 

 since at that time all products of assimilation were assumed to be deposited 

 in that form. Ten years later, BROWN and MORRIS (1893) repeated SACHS'S 

 experiments, and confirmed them in all essentials, although they obtained 

 smaller values than he did ; but they also showed that only a fraction of the 

 products of assimilation occurred in the form of starch. In one case, for example, 

 Helianthus showed an increase in weight of 8-566 g. per sq. metre in twelve 

 hours (in addition to 4g. translocated) but of this amount only i-4g. was starch, 

 all the rest was sugar. Similar results are recorded in the works of A. MEYER 

 and in several other memoirs which were published in the interval. MEYER'S 

 studies on the nature of the sugars occurring in the leaf are therefore especially 

 valuable, and for such investigations he found the leaves of Tropaeolum to be 

 particularly adapted. Without discussing the methods employed we may give 



