22 NATURAL SCIENCE. July, 



the atmosphere was shown to be incorrect by Meyer (1885-6), who 

 found that starch is readily formed by the mesophyll chloroplasts 

 when the leaves are floated on solutions of dextrose, levulose, cane- 

 sugar, galactose, and even mannite and glycerine. 



Baeyer's suggestion that the first formed product of the carbon 

 assimilation is formaldehyde — a compound which undergoes easy 

 condensation into sugar-like substances — has been considerably 

 supported by recent work on the sugars, notably by Emil Fischer's 

 syntheses of dextrose and levulose, although attempts to feed 

 chloroplasts with aldehyde or its derivatives have met with only 

 partial success. 



The most recent and at the same time one of the most im- 

 portant contributions ever made to the study of plant-metabolism, is 

 that of Messrs. Brown and Morris, communicated to the Chemical 

 Society in April last. In their paper they deal with the occur- 

 rence of starch, diastase, and sugars in leaves, and the work will 

 undoubtedly take high rank in the annals of plant physiology. 



Sachs, in 1884, endeavoured to determine the weight of starch 

 produced during the day by estimating the dry weight of two definite 

 equal areas of the leaf, taken symmetrically from opposite sides of 

 the midrib, one in the morning and the other in the evening after the 

 day's exposure to sunshine. The assumption was made that the 

 differences in dry weight were due to gain in starch, and the 

 results showed in the case of a sunflower an increase of 9*14 

 grains per square metre in ten hours. Similar experiments 

 carried out on plucked leaves with their petioles immersed in 

 water yielded greater increase in weight, owing to the dissolution 

 products of starch not being able to flow into the stem under these 

 circumstances. 



Messrs. Brown and Morris point out that this method merely 

 indicates the total assimilated products which are manufactured under 

 the given circumstances, and not necessarily the amount of starch 

 produced. Sachs thought otherwise, as he assumed that all the 

 assimilated products of the leaf must at one time or other pass through 

 the form of starch in the chloroplasts, an assumption for which there 

 is no evidence. These observers bear testimony to the applicability 

 and accuracy of Sachs' dry weight method of estimating the rate 

 of assimilation, and go on to determine the amount of starch which 

 is present at any one time in the leaf, and the proportion which it 

 bears to the total products of assimilation. 



Leaves were rapidly dried at 75-80 C, or killed by the action of 

 chloroform vapour, and then finely powdered and the fats and chloro- 

 phyll extracted with ether. Ten grams of the powdered leaves were 

 then taken and extracted with alcohol ; the starch in the powder was 

 then gelatinised by hot water, and finally determined by estimation 

 of the maltose and dextrin produced after the action of diastase upon 

 it for about two hours. 



