128 PLANT PHYSIOLOGY. 



B. Heliotropism. 



It is a familiar fact that the green parts of most plants 

 respond strongly to one-sided light, a phenomenon known as 

 heliotropism (or phototropism) . It is needful to examine its 

 nature with more exactness. 



93. How are growing stems, leaves, and roots influenced in di- 

 rection by the direction of the light falling upon them ? 



Answer by Experiment 55. 



EXPERIMENT 55. This may be tested by exposing germinating 

 seedlings, growing in water, to one-sided light of constant direction. 

 Prepare a tumbler-germinator like that used in Experiment 13 

 (Fig. 13), except that no black paper is to be used and the absorbing 

 fabric must be near the top of the tumbler (to allow room for the 

 roots) and must allow free access of light upon one side. On the 

 fabric place 5 or 6 soaked mustard-seeds, fill to the proper height 

 with tap-water ; keep in darkness until steins are 3 to 4 cm. high, 

 then cover with a hood in which is a hole 2 or 3 cm. in diameter 

 at the height of the seeds. Place with the hole turned to strong 

 light, and observe effect upon growth of the stems, leaves, and roots. 



In general what is the difference in the direction taken by 

 these three parts with respect to light direction ? 



When Geotropism and Heliotropism act together but in differ- 

 ent directions upon a sensitive part, what is the result? 



Answer by observation of Experiment 55. 



For logical completeness, there should properly be tried here, as in 

 the case of Geotropism, the correlative experiment of growing similar 

 plants with light direction eliminated, either by its removal altogether 

 or by its .neutralization through use of a revolving clinostat. Since 

 growth in darkness introduces unusual conditions and effects, the use 

 of the clinostat is much the best. The clinostats already described under 

 section Si are of course ample for this purpose ; the clock clinostats re- 

 volving horizontally will carry easily a five-inch pot, and very easily the 

 germinator used in the above experiment. 



For experiments upon heliotropism in adult shoots, Tropoeolum, the 

 Garden Nasturtium, is particularly good. 



94. Do the reception of the heliotropic stimulus and the growth 

 response necessarily occur at the same place? 



Answer by Experiment 56. 



