HELIOTROPISM AND HYDROTROPISM. 129 



EXPERIMENT 56. This may be answered if a plant can be found 

 in which, when the growth zone is exposed to light but certain 

 other parts are kept in darkness, no heliotropic movement occurs. 

 In a small pot of earth filled to the brim (so that the edge of the 

 pot cannot throw a shadow) sow a dozen oats, and place in darkness 

 until they appear just above the surface. Prepare six very tiny cy- 

 lindrical caps of tin-foil, i cm. long and just wide enough to fit over 

 the tips of the young plants (they may be moulded on a large head- 

 less pin); quickly taking the pot from the darkness, slip the caps over 

 six of the tips, thus covering them from light, and leave the others 

 uncovered. Set tiie pot where it will receive very one-sided light 

 and observe results. 



95. What rays of the white light are most efficient in inducing 

 heliotropic curvatures ? 



Answer by Experiment 57. 



EXPERIMENT 57. This may be determined by noting towards 

 what pure color of light heliotropically-sensitive plants most quickly 

 turn. The colors may be supplied by screens of glass or gelatine, 

 though the objections already mentioned (under their use in growth 

 studies, page 108) apply to them for this purpose. Hence the 

 pure-color chambers earlier described (page 108) should be used. 

 The apparatus should be set vertically and a layer of compact sphag- 

 num placed along the side (in this position the bottom) of the 

 smaller pots, and on the moss a few oats. The whole is to be kept 

 in darkness until the oats are 2 cm. high, when the color screens are 

 to be put in place (the flasks are to be kept in position by gummed 

 paper attached to their edges and to the felt paper of the glass 

 cover), and exposed to a strong light for a few hours, when the 

 comparative bending under the different colors may be determined, 

 and measured. 



Another class of light effects, distinct from the directive effects of 

 heliotropism, are the nyctitropic, including most ' sleep " movements. 

 These are very easy to experiment with, using Clover, Oxalis, or Mimosa 

 plants. Of still another character is the striking movement of chloro- 

 plasts towards or away from light, according to its intensity. Another 

 class includes the light-protective movements shown by many Legumi- 

 nosse, and other plants. 



C. Hydrotr.opism. 



The tendency of roots to seek moist places is familiar and 

 of much ecological importance. 



96. To what extent are roots sensitive to moisture? 



Answer by Experiment 58. 



