34 



PHYSIOLOGY OF NUTRITION. 



barley seedlings as research material. A good number of small 

 flower-pots are filled with garden soil, barley grains are sown 

 in each, and the pots are then put in the dark. When the 

 plumules have reached a length of say 2 cm., we place one 

 flower-pot before the window of a room with a north aspect, 

 and in which the temperature is 6C. Another pot is placed in 

 a neighbouring room, in which the thermometer indicates 20 C. 

 The plants are thus exposed to similar conditions of illumination, 

 but to different temperatures, and it is found that the greening 

 takes place far more slowly at 6C. than at 20 C. At 30 C. the 

 formation of chlorophyll proceeds somewhat more rapidly than 



at 20 C., at 37 C. 

 slower again, and at 

 45 C. no greening 

 takes place at all. To 

 expose plants to tem- 

 peratures of 30, 37, 

 and 45 C., we put 

 them into thermostats 

 heated to the required 

 temperatures. (A 

 drawing and descrip- 

 tion of a suitable 

 thermostat will be 

 found in the second 

 section, in connection 

 with researches on 

 root pressure.) It re- 

 mains to be noted that 

 the seedlings, before 

 being illuminated, must 

 be kept for some time in places of accurately known temperature, 

 so that they, and also the flower-pots and the soil in them, may 

 accommodate themselves to these temperatures. Judging from 

 what was said above there are temperature-minima, -optima, and 

 -maxima for the process of greening. The positions of these are 

 by no means the same in different plants. 2 



Lastly we will prove that the process of greening in the light 

 cannot take place in absence of Oxygen. We fill two retort- 

 like vessels (a and b) with distilled water which has been boiled 

 and allowed to cool again in closed vessels, place in the water a 



FIG. 11. Apparatus for the culture or plants in, a 

 space devoid of Oxygen. 



