THE CARBOHYDRATES OF THE ANGIOSPERM LEAF 3 



contained reserve materials of the seed within their other 

 tissues, could form starch under the above-mentioned con- 

 ditions. Both Meyer and Bohm demonstrated the exist- 

 ence of starch in leaves, which had been completely 

 depleted of it, when they were subsequently cut into strips 

 and floated on the surface of sugar solutions. Experiments 

 of this type have proved that both the chloroplasts of the 

 mesophyll and the amyloplasts of tissues devoid of chloro- 

 phyll are able to form starch from sugars. These results 

 however, do not negative the possibility of direct starch 

 formation, though they render it highly improbable. 



Brown and Morris first of all turned their attention to 

 the determination of the starch in the leaf, and the pro- 

 portion it bears to other products of assimilation. To 

 obtain the material for comparative experiments they 

 adopted Sachs's half-leaf and template method, thereby 

 obtaining similar and equal areas. In the case of Helian- 

 tTius annuus, it was shown that the probable error of the 

 method amounted to about 1 per cent., the leaf tissue of 

 the right-hand halves weighing 40*53 grammes per square 

 metre, the remaining halves giving 40*96 grammes. 



As an example of the degree of accuracy of their chemical 

 manipulations the following experiment is quoted : Working 

 with 10 grammes of dry leaf powder, two similar analyses 

 afforded 6-408 per cent, in one case, and 6-545 per cent, of 

 starch in the other. The leaves were those of Tropceolum 

 majus, picked after a sunny day and quickly dried. They 

 were usually found to contain from 3-5 to 7-5 per cent, 

 of starch calculated on the dry weight. Leaves which 

 were kept for . sixty-three hours in darkness, with their 

 petioles in water, showed a loss in starch amounting to over 

 two -thirds of that present ^at the time of picking,, and 

 are therefore rapidly depleted of their stored starch under 

 such conditions. 



