TRANSACTIONS OF SECTION D. 813 



In a living plant, placed under favourable conditions, the starch of the chloro- 

 plasts disappears with much greater rapidity than it does if the leaf has been 

 previously treated by some method which arrests the vitality of its cells, but does 

 not affect the activity of the contained diastase. At first sight this fact, upon 

 which Wortmann lays great stress, seems to negative the idea that the dissolution 

 of the starch is due to hydrolysis by diastase. Recent experiments carried out by 

 the author and his colleague have, however, convinced them that the action on 

 the starch is really conditioned by the diastase, but that the action is enhanced 

 and rendered continuous by the ability of the living elements of a cell to seize 

 upon the chemical products of hydrolysis, and to remove them from the sphere of 

 action by passing them into adjoining cells. 



The amount of diastase present in leaf-cells is found to be subject to periodic 

 fluctuations. Like the fluctuations in the amount of starch, those of diastase 

 appear to be governed by the intensity of illumination. The conditions, however, 

 which are favourable to a decrease of starch are just those which are favourable 

 to an increase of diastase, and vice versa ; so that we find more diastase 

 in a leaf during the night than during the day. This variation really appears 

 to depend upon the rate of nutrition of the cells. Whilst these are sup- 

 plied with an abundance of material in the form of freshly assimilated 

 sugars they elaborate little or no diastase, but when the supply of these sugars 

 falls oft' diastase is secreted for the purpose of dissolving the excess of carbohydrate 

 which has been stored up as starch. The secretion of diastase by the leaf-cell, and, 

 in fact, by every starch-containing cell, is a phenomenon of partial starvation, and 

 IS necessary for the autophagy of the cell. A precisely similar phenomenon had 

 been previously observed during the germination of certain endospermous seeds.^ 

 This is a principle which will probably be found of general application to all cases 

 of secretion of enzymes both by animal and vegetable organisms. 



Certain experiments are then discussed which were planned with a view to 

 ascertain the nature of the sugars existing in the leaf, their variations in amount 

 at difierent periods, and the relation which each sugar bears on the one hand to 

 the first product of assimilation, and on the other to the starch deposited within 

 the chloroplasts. The leaves of Tropmolum inajus were selected for this portion 

 of the inquiry. 



The only sugars which could be detected in any quantity were cane-sugar, 

 dextrose, levulose, and maltose. Only mere traces of the pentoses could be found. 



The results yielded by a study of the quantitative variations in the amount of 

 the sugars and starch of leaves, when these have been placed under determinate 

 conditions, are decidedly opposed to the view that either dextrose or levulose is the 

 first sugar formed in the assimilative processes. Cane-sugar appears to be the first 

 distinctly recognisable carbohydrate which is formed. There is every reason to 

 believe that this cane-sugar, which may be regarded as the starting-point of all 

 the metabolic changes taking place in the leaf, accumulates in the cell-sap of the 

 leaf parenchyma when assimilation is proceeding vigorously. When the concen- 

 tration passes beyond a certain point starch commences to be elaborated by the 

 chloroplasts at the expense of the cane-sugar. This starch forms a more stable 

 reserve substance than the cane-sugar itself, and is drawn upon when the more 

 readily metabolised cane-sugar has been partially used up. 



5. On Ci/tological Differences in Momologous Organs. 

 By Professor G. GiLSON, of Louvain. 



Remarkable differences may be detected in the minute structure of certain 

 organs which are considered as unquestionably homologous ; and it would not be 

 without interest to know to what extent these differences may lead the morphologist 

 to modify his views as to this homology. 



A striking instance of such differences is found amongst the nephridia or 

 segmental organs. 



' Cf. Brown and Morris, Jour, of Chem. Sac, 57 (1890), p. 458. 



