198 CHLOROPLASTS. [CH. VIII 



same material. The sieve is now hung up so that the 

 bottom makes an angle of about 50° with the horizon. 

 As the roots emerge they leave the vertical and grow 

 along the moist surface of the sieve. We find that 

 cereals such as rye or barley answer well. The only 

 difficulty is to provide a suitable atmosphere in which to 

 hang up the sieve. If the air is too dry the roots wither 

 before they have time to bend ; if too damp the surface of 

 the sieve does not supply a sufficiently strong contrast to 

 the surrounding air. A greenhouse atmosphere answers 

 fairly well, or, as Sachs recommends, a large cupboard or 

 dark room of which the floor is occasionally watered. 

 According to the same authority the air should be in such 

 a condition that the difference between the wet and dry- 

 bulb thermometers is 1-5°— 2'0° K (2°— 2-5° C). We 

 find it a good plan occasionally to squii't with water the 

 surface of the sieve. 



(248) Movement of chloroplasts. 



We find that the leaves of Oxalis acetosella^ give good 

 results. Ten or twelve leaves are taken from a plant, and 

 after the stalks have been cut short off beneath the 

 pulvini they are placed floating in water. Half of the 

 number in one dish are exposed to bright sunshine, the 

 rest remain in dull diffused light. After two hours they 

 may be examined by preparing surface sections of the 

 spongy parenchyma. In the sunned leaves the chloroplasts 

 are in the " profile position," that is, they lie against the 



1 This plant is recommended by Stahl, Botaniscke Zeitung, 1880. 

 Stahl's figure of Oxalis is copied in Frank's Lehrbuch, p. 289. 



