28 THE WORK OF THE LEAF [chap. 



Though starch is manufactured in the leaf it does 

 not accumulate there, in the darkness it is moved wholly 

 or partially away and stored in some other part of the 

 plant : were it not so, leaves would become thick and 

 swell with the stores of manufactured starch. The 

 transference of the starch is effected by a similar 

 mechanism to that which utilises the starch in a seed 

 for the use of the distant-growing parts of the plant ; 

 the leaf cells secrete an enzyme capable of dis- 

 solving starch and transforming it into sugar, which 

 being soluble in water can move about in the plant. 

 To show the presence of this diastase we must gather 

 some young and active green leaves in the early 

 morning, or after they have been shut up from the light 

 for some time, as in a previous experiment. Now dry 

 these leaves very quickly but at a low temperature, best 

 of all by exposing them over strong sulphuric acid in a 

 desiccator from which the air has been pumped. Under 

 these conditions the leaves will dry very rapidly and 

 their enzymes will suffer no change; when dry, powder 

 the leaves finely and introduce a little of the powder into 

 a starch paste solution as before. The starch paste 

 solution will rapidly become limpid, will lose its power 

 of turning blue with iodine, and will finally give a 

 reaction for sugar with Fehling's solution. Clearly the 

 green leaf contained an enzyme which acts on the starch 

 paste like that which was present in the extract from 

 germinated barley — a diastase transforming starch into 

 soluble sugar capable of diffusing out of the leaf into 

 such parts of the plant as may require it. So funda- 

 mental is the process of assimilation that yet another 

 experiment may be described to illustrate the action. 

 For this purpose it is necessary to have at hand a plant 

 or plants possessing a considerable number of large 

 leaves : the vine, hop, or sycamore are among the most 



