NUTRITION, STORAGE, AND RESPIRATION iii 



Synthesis. Or they may appear as colourless leiikoplasts, in cases where 

 starch is formed in parts shut off from the Hght (Figs. 79, 8[, .^l-fi). Here 

 the starch-grains arc commonly of large size, and tlie leukoplasts being 

 relatively small and colourless, they have frequently been overlooked. 

 Nevertheless accurate observation has shown that where starch is 

 formed, either leukoplasts or chloroplasts are constantly present, and 

 precede them. It thus appears that the capacity for converting sugar into 

 starch resides in the plastids. 



But when the insoluble starch is again required for physiological 

 use, it is converted back into sugar, in which form it can be transferred 

 to the point where it is required. A movement through the tissues 





,<?• 



Fig. 81. 

 A-E, Leukoplasts from tuber of Phajiis), showing various stages of development 

 of starch grains. 1-4. Various stages of the corrosion of starch-grains in gemninating 

 Barley, (.\fter Strasburger.) 



would obviously be impossible for the solid grain. The change happens 

 quickly in the case of the grains included in the chloroplasts, and a 

 few hours in the dark suffices for their re-solution. The details of 

 the change are best observed 'in the large storage-grains such as those 

 of the Barley. It is found to depend upon the action of Diastase, an 

 enzvme or ferment widely distributed in plant-tissues where starch is 

 present, and is being used up : as in germinating grains. This sub- 

 stance can be extracted from tissues that contain it, by glycerine, and 

 precipitated from the solution by alcohol. If the precipitate be 

 collected and re-dissolved in water, a solution is obtained which still 

 keeps its properties ; but these are destroyed on exposure to a tempera- 

 ture of 75° C. By its reactions diastase is shown to have much in 

 common with proteids. If n solution of diastase be added to starch- 



