MOVEMENTS OF GROWTH. 415 



to screen them from the light. Other potatoes we place in a box 

 covered with a sheet of glass. The two boxes are placed in front 

 of the window in a heated room with a north aspect. No water 

 at all is supplied to the tubers. During the winter the potatoes 

 germinate, but whereas the shaded ones produce fairly long shoots, 

 the shoots from the illuminated ones remain short and compact in 

 appearance. We may continue the investigation on into the 

 summer, and it is always found that the development of the first 

 internodes of the shoots arising from the potatoes can only proceed 

 normally in darkness (under ordinary conditions in the soil). If 

 now and then we rub down on a grater one tuber from each series, 

 treat the pulp in each case with water, filter, and test the filtrate 

 with Fehling's solution for sugar, we shall find that the one which 

 had been shaded contains a large quantity of sugar, while that 

 which was illuminated contains none at all or only traces. With 

 this want of suitable plastic material in the illuminated tubers is 

 clearly connected the weakly growth of their shoots. I was the 

 first to establish the fact x that potato tubers germinating in the 

 li^ht do not contain sugar ; Ziegenbein (Pringsheim's Jahrbucher, 

 Bd. 25) pursued the matter further. 



It is further noteworthy that potatoes placed under the influence 

 of the light gradually become green. If we examine under the 

 microscope delicate transverse sections from a potato which has 

 become green, we find immediately below the skin cells which 

 contain chlorophyll bodies with starch enclosures. These chloro- 

 phyll bodies are produced under the influence of the light from 

 colourless amyloplasts which the potatoes contain. 



The appearance of the shoots of germinating potatoes is quite 

 different if they are supplied with water. We fill a few plates 

 with moist sand, and keep the sand constantly moist. A few 

 potatoes are pressed into the sand upright, with their morpho- 

 logical base downwards. Some of the plates are covered with a 

 large bell-glass, the others are placed in a large zinc box. The 

 shoots of the illuminated specimens, especially their first inter- 

 nodes, develop as short thick structures provided with numerous 

 scale leaves, but furthermore roots are formed, which penetrate 

 into the sand. Here again the shoots of the tubers kept in the dark 

 attain a considerable length while remaining small in diameter, 

 and their root primordia burst out. In moist air, therefore, many 

 roots always develop on the potato shoots ; in dry air they do no* 

 develop at all, or only to a very limited extent. Moreover, moist 



