458 PHYSIOLOGY OF GROWTH. 



atus, and to study the behaviour of many plant organs under 

 different forms of rotation, we make the following experiments. 

 We put the clinostat in front of the window, and on the disc (see 

 Fig. 152) rotating horizontally about a vertical axis, place a 

 flower-pot containing a few very sensitive heliotropic seedlings 

 (Sinapis, Lepidium) which have just emerged from the soil, and 

 which have hitherto been kept in the dark. As long as the rota- 

 tion continues, these seedlings undergo no heliotropic nutations, 

 in spite of the one-sided illumination, while similar objects not 

 rotated all curve over towards the window. 



In order to eliminate, in experiments with seedlings, etc., the 

 one-sided action of gravity, the axis of rotation must be accurately 

 horizontal (see Fig. 148). 



Rotation in the vertical plane taking place at the rate of one 

 revolution per ten to fifteen minutes is slow enough to prevent any 

 effect of centrifugal force. Above all, it is, however, exceedingly 

 important, in order to secure uniform rotation, that the loading 

 of the axis shall be uniform all round. This can be effected by 

 means of the pin h and the weight i (see Fig. 148). The 

 seedlings themselves, e.g. maize, pea, or bean seedlings, grown in 

 sawdust, are placed in a drum constructed out of thin sheet zinc. 

 This is put in the pot-holder in place of a flower-pot, and suitably 

 fixed. The base of the drum is formed by a piece of soft wood, 

 so that the long pins which carry the seedlings may be fixed with- 

 out difficulty. The sides of the drum are covered inside with 

 several layers of moistened blotting-paper, as also the non-growing 

 parts of the seedlings. On the base of the drum we spread moist 

 cotton wool, which is held in place by means of small pins. It is 

 also very convenient to experiment as follows. We connect up 

 one end of a shaft about 80 cm. in length with the clockwork of 

 the clinostat, while the other end rests on friction rollers. We 

 now slip a tightly-fitting cork over the shaft, so that it can be 

 rotated like a wheel in the vertical plane. The seedlings are 

 then fixed on the periphery of the cork with two pins each, so 

 that the weight is distributed as evenly as possible. Under the 

 cork stands a reservoir of water, so that the plants at each revo- 

 lution of say twenty minutes dip for a while (one to two minutes) 

 into water. By covering with a glass box, provided with corre- 

 sponding slits for the shaft, the air in the neighbourhood of the 

 seedlings is kept moist. The whole apparatus is put in a dark 

 chamber. 



