812 REPORT— 1893. 



maltose and dextrin ty hydrolysis with malt-diastase. The products of hydro- 

 lysis, when quantitatively determined by means of the polarimeter and hy their 

 cupric-reducing power, are a measure of the starch from which they are derived. 

 In applying this method to a critical examination of the intiuence of surrounding 

 conditions on the formation of leaf-starch, precautions have to be taken to arrest 

 the vitality of the leaf at the moment of its separation from the plant. 



The results of many varied experiments point consistently to the conclusion 

 that starch is not an essential link in the chain of chemical products begiuDing with 

 the inorganic materials, carbon dioxide and water, and ending with the form in 

 which the assimilated material passes out of the leaf. No proof, direct or indirect, 

 could be obtained of that rapid building up and breaking down of the starch which 

 must be going on in every assimilating cell of starch-producing plants, if all the 

 metabolites of a carbohj'drate nature have to pass through the form of starch. 



It appears almost certain that the deposition of starch within the chloroplast is 

 governed by the same laws as those which regulate its deposition in the amyloplast 

 of non-assimilating cells, and that this is primarily conditioned by the rate at which 

 the soluble carbohydrates are supplied to the cell. Starch is only deposited in the 

 chloroplasts or amyloplasts when the supply of this soluble material is greater than 

 the local requirements of the individual cell, or its immediate powers of trans- 

 location. 



"When leaves are placed under conditions favourable for the depletion of their 

 starch, the starch grains which are embedded in the chloroplast of the stomatic 

 guard-cells are very much more stable than those of the palisade-cells or of the 

 spongy parenchyma. This is due to the comparative physiological isolation of the 

 guard-cells, an isolation brought about by the more or less complete cuticularisa- 

 tion of their walls, by their slight lateral attachment to the cells of the epidermis, 

 and by the fact that they have no immediate connection with the mesophyll and 

 the conducting sheaths surrounding the vascular bundles. The guard-cells, in fact, 

 do not belong to the general republic of the assimilating cells of the parenchyma, 

 but are autonomous. By the aid of their chloroplasts, which are always present, 

 they assimilate only for themselves ; and, on the other hand, when the conditions 

 of illumination render assimilation no longer possible, the carbohydrate which they 

 have stored up in the form of starch is only drawn upon for their own individual 

 requirements, and is not passed into the stem, as are the products of dissolution of 

 the starch formed in the parenchyma. 



When it is considered how important it is to the plant that these guard- 

 cells should be kept in a high state of working efficiency, and no matter what the 

 variations in illumination maj' be, the advantages derived from the isolation and 

 autonomous nature of these cells are very apparent. 



A great number of experiments were made in order to determine the mechanism 

 by which the starch is redissolved in the living cell of the leaf parenchyma, and 

 rendered available for the requirements of the plant. It was found, notwithstand- 

 ing the statements of AVortmaun to the contrary, that leaves always contain far 

 more diastase within their tissue than is necessary for the complete dissolution of 

 the starch at any time present. A method was devised for the relative determina- 

 tion of the amount of this enzyme in the leaf, and the results of a great number of 

 such determinations were given. 



The leaves of the leguminosse are pre-eminently rich in this enzyme, those of 

 Fisian sativum having a diastatic activity equal to from one-half to one-third that 

 of an average barley-malt. 



The full diastatic efl'ect of leaves is onlv apparent when the tissue, previously 

 dried at a low temperature, is brought into actual contact with the starch solution. 

 Mere infusions of the leaf are not \ ery active, and if it contains any considerable 

 amount of tannin, which is sometimes the case (e.ff., Hop, and Hydrocharis 

 mvvsus-rancB), the leaf infusion has no hydrolytic action at all on starch, owing to 

 the enzyme having been prevented from going into solution by the tannin. 



The products of the hydrolysis of starch under the action of leaf-diastase have 

 been carefully studied, and ai-e found to be identical with those formed under simi- 

 lar conditions by malt-diastase. 



