THE FOOD OF PLANTS. 55 



as follows : Two crystallising glasses are selected (see Fig. 19, a 

 and a'), whose well-ground edges fit closely when put together. 

 The edges of the glasses are now smeared with tallow, and starch- 

 free leaves of Cucurbita Pepo or Vitis vinifera taken from shoots 

 which have been kept in the dark for some time, so that they are 

 starch-free, are placed between the edges in such a way that the 

 tip of the leaf b is within the closed space, the base of the leaf 

 and the leaf-stalk being outside. The lower glass contains potash 

 solution. By gentle pressure the space between the glasses, say 

 000 c.c., is made air-tight. The leaf-stalk dips into a small vessel 

 containing water. The whole apparatus is now placed under a 

 large calibrated bell-glass of about 4,000 c.c, capacity. It rests on 

 flat pieces of marble, and dips into water. By means of the bent 

 tube R, we suck up into the bell-glass 200 c.o. of water, and then 

 lead in pure Carbon dioxide to replace this quantity of water. 

 The apparatus is now exposed for a few hours, possibly shaded, to 

 direct sunlight. It is found, finally, that the portion of the leaf 

 kept in the air charged with about 5 per cent, of Carbon dioxide, 

 is very rich in starch, while no starch is produced in the terminal 

 part of the leaf. 



Experiments to demonstrate that conditions of temperature 

 influence the formation of starch in the chlorophyll are best con- 

 ducted in autumn or winter. Elodea plants freed from starch by 

 being kept for some time in the dark, are placed in spring water 

 in two rooms having the same aspect. One room is kept at a 

 temperature of about 6C., the other at a temperature of about 

 20 C. The temperature of the water in the one case is kept 

 constant at 6 C. (if necessary by introducing fragments of ice), in 

 the other case at 20 (if necessary by adding warm water). From, 

 time to time (say every 30 minutes) we test the leaves for starch^ 

 and find that although the plants have been exposed to similar 

 conditions of illumination, larger quantities of starch have formed 

 more rapidly at the higher temperature than at the lower. 3 



1 See Godlewski, Flora, 1873, p. 382. 



2 See Moll, Landtcirthschaftl. Jahrb., Bd. 6, p. 345. 



3 Literature respecting starch formation : Sachs, Botan. Zcitnng, 1862, Ny. 

 44; the same, 1864, Nr. 38 ; Arbeiten d. botanischen Instituts in Wilrzburg ,*&&.. 

 3, H. 1 ; G. Kraus, Pringsheim's Jahrbilcher, Bd. 7, p. 511 ; Nagamatsz, 

 Jteitraye zur Kenntniss d. C hlorophyllf unction, Dissertation, Wiirzburg, 1886. 



