50 PHYSIOLOGY OF NUTRITION. 



supplied to them from outside. Fresli pieces of Iris germanica 

 leaf, about 10 cm. in length, were placed, without removal of the 

 layer of wax, in a 20 per cent, solution of cane-sugar, contained 

 in a shallow glass dish. The pieces of leaf floated on the fluid, and 

 one of their surfaces did not come into contact with it at all. The 

 vessel was covered with a sheet of glass, but between this and the 

 rim of the vessel was placed a piece of cork so as to secure access 

 of air. The material remained in contact with the cane-sugar 

 solution for more than a week at a moderate temperature, and in 

 darkness. At the beginning of the experiment, I tested some of 

 the pieces of leaf macroscopically for starch ; there was none 

 present. Pieces of leaf which had remained for eight days in con- 

 tact with the sugar solution gave a distinct starch reaction. It is 

 to be observed that the pieces of Iris leaf, before being placed in 

 the Iodine solution, must be treated for rather a long time with 

 warm alcohol, to secure complete removal of their chlorophyll. 

 The investigation may also be conveniently made with leaves of 

 Nicotiana tabacum. Early in the morning a half leaf of the 

 plant is tested macroscopically for absence of starch, the other 

 half being laid in cane-sugar solution. After leaving this in the 

 fluid for several days in the dark, it is found to contain starch 

 in abundance. 



1 See Sachs, Handbuch d. Experimentalphysioloyie d. Pftanzen, 1865, p. 326. 



2 Artb. Meyer, Botan. Zfitung, 1885, Nr. 27. 



3 See also Saposchnikoff, Berichte der Deutschen botan. Gesellschaft, Bd. 9. 

 Further see Brown and Morris, Journal of the Chem. Soc., May, 1893. 



4 See Sachs, Arbe.iten d. botan. Institnts in Wiirzburg, Bd. 3, and Saposcli- 

 nikoff, Berichte d. Deutschen botan. Gesellschaft, Bd. 8. 



5 See Bb'hm, Botan. Zeitung, 1883 ; A. Meyer, Botan. Zeitung, 1885, Nr. 27, 

 and especially Botan. Zeitung, 1886, Nr. 5. 



16. The Dependence of Starch For nation in Assimila tion on 

 External Conditions. 



We sow some Phaseolus seeds in loose garden earth contained 

 in flower-pots, grow the plants in darkness till the cotyledons 

 have been robbed of a very considerable part of their stored 

 reserve material, and now examine the primordial leaves macro- 

 chemically, after Sachs' method, or microchemically by treating 

 thin transverse sections with chloral hydrate and iodised solution 

 of "Potassium iodide (see 14). Starch is not found in the meso- 

 phyll cells. If we now leave the plants for a few days exposed to 



