162 



PLANT PHYSIOLOGY 



organic compounds of the other elements such as nitrogen, 

 sulphur, phosphorus, etc., that enter into the composition of the 

 protoplasm. 



Of all the carbohydrates sjmthesized in the plant, it is most 

 easy and convenient to observe the formation of starch, for it 

 accumulates in grains that may be seen under the microscope. 

 Starch may be detected also by the well-known iodine reaction. 

 By staining with IKI solution, the formation of starch grains in 

 the green plastids of plants may be easily followed under the 



microscope especially in the thin leaves 

 of aquatic plants (Fig. 46), in mosses, 

 and in the cells of algae. With suffi- 

 cient illumination, this process will take 

 place under the microscope, and the 

 increase in size of the starch grains in 

 the chloroplasts may be seen directly. 

 In direct sunlight, 5 min. is sufficient 

 for the formation of starch. Even with 

 the comparatively weak light of a 

 petroleum lamp of 100 foot-candle 

 power, Famintzin as early as 1860 

 observed the formation of starch in the 

 cells of Spirogyra in 30 min. after the 



Fig. 46.— Starch grains in beginning of the experiment. Another 

 the chloroplasts of Eiodea convenient method for determining the 



{after Mohsch). u • .i n 



accumulation oi starch is the well- 

 known Sachs test. A leaf, having been previously exposed to 

 light, is decolorized by means of alcohol and afterward treated 

 with a solution of iodine dissolved in potassium iodide. If 

 separate areas of the leaf are shaded with opaque paper prior to 

 the experiment, there will be produced light figures on the general 

 iodine-stained background of the leaf. The relationship between 

 the amount of light transmitted and the accumulation of starch is 

 so close that by means of this method it is possible even to print 

 in the leaves portraits from negatives just as on photoprint 

 paper (Fig. 47). 



Starch is the most easily detectable product of assimilation. 

 This does not mean, of course, that it is the first product, for it 

 represents a far too complex structure made up of large mole- 

 cules. Its production in the leaf is preceded by the formation 



