98 SUMMARY OF CURRENT RESEARCHES RELATING TO 



visiblo effect of reduced vitality is a change in the green pigments. 

 The chlorophyll is changed into colourless products, or, by the 

 presence of a weak acid, into a very stable brownish-green, which 

 resists further change ; thus giving rise to the production of bright 

 yellows or dull browns, as in the alder. It is probable that under 

 some conditions the chlorophyll in leaves is changed by the action of 

 light into a red substance ; the change depending on a certain amount 

 of reduced vitality, as well as on little-understood conditions varying 

 in diftcrent kinds of plants. 



The next series of changes is best studied in the case of those 

 leaves which in the first instance turn to a bright yellow, and depends 

 mainly on the production of deeply coloured pigments by the oxidiza- 

 tion of tannic acid and other more or less colourless substances ; the 

 difference in tint depending on the nature of these substances. Thus 

 the tannic acid in the yellow oak leaves changes into a bro\vn substance ; 

 whereas the quinotannic acid in yellow beech leaves changes into an 

 orange-brown. Fine effect very much depends upon the production of 

 each special tint in a fairly pure state. This seems to be influenced 

 by the character of the weather. It is also important that the half- 

 dead leaves should hang long on the trees, so as to devclope their full 

 colouring before being blown off by the wind. 



Formation and Fermenting Action of Diastase.* — W. Detmer 

 states his opinion that acids act directly on the ferment, increasing its 

 activity, and consequently facilitating the conversion of starch into 

 sugar. This takes place as the result of peculiar motions of the mole- 

 cules and atoms, which increase the molecular movements of diastase. 

 The physiological importance of this process consists in the promotion 

 of the growth of the cell-wall, connected with the increased formation 

 of sugar incited by the acids of the cell-sap. The chlorides of 

 potassium and sodium accelerate the action of the ferment when the 

 solution is only slightly acid, but retard it when the acid reaction is 

 strong. In the absence of the ferment they exercise, within a short 

 time, no perceptible influence on starch-paste. Detmer believes that 

 the action of the chloride depends on the setting free of hydrochloric 

 acid by the action of organic acids ; and this he considers to be the 

 function of chlorides in plants. The conversion of starch into sugar 

 takes place at 10-5° C. ; slowly cooling, even to — 10°, followed by 

 a quick rise of temperature, docs not diminish the fermenting power. 

 Higher temjieratures destroy the ferment. The process continues 

 when the reaction is slightly alkaline. The formation of diastase is 

 not sensibly affected by light ; the retarding influence of light on 

 starch is probably due to increased respiration, or a greater recon- 

 version of sugar into starch. Experiments made in January indicated 

 the entire absence of sugar and diastase in the tubers of the potato ; 

 after germination commences, diastase is formed in continually in- 

 creasing quantities. In the cells of the higher plants no diastase is 

 produced in the absence of free oxygon. 



* Detmer, W., ' Pflanzenphys. Unters. iiber Fermenfbildung u. fermentative 

 Processe,' 50 pp., Jena, 1884. See Bot. Centialbl., xix. (1884) p. 164. 



