408 PHYSIOLOGY OF GROWTH. 



cotyls of the seedlings have turned towards the rays of light 

 passing through the slit, while the roots on the contrary have 

 turned away from the light. In Fig. 157 is depicted a seedling 

 whose hypocotyl has performed a positive heliotropic nutation, 

 while its root has experienced negative heliotropic nutation. 



Only comparatively few plant structures are negatively helio- 

 tropic. Positive heliotropism on the other hand is very common. 

 Good research material is found in seedlings of Phaseolus multi- 

 florus, Yicia sativa, and Lepidium sativum, grown in the dark, 

 We cultivate the plants in small flower-pots, filled with loose 

 garden earth, and when the epicotyls or hypocotyls as the case 

 may be are in vigorous growth, expose them to unilateral illumi- 

 nation in the manner above described. The heliotropic nutations 

 are soon exhibited, and often proceed till the ends of the shoots 



FIG. 158. Heliotropic chamber. 



are parallel with the incident rays of light. The heliotropic 

 sensitiveness of different structures is, however, by no means the 

 same. For example, the epicotyls of Vicia sativa, and plumules, 

 say 2 cm. in length, of Triticum vulgare, are very sensitive, 

 while the epicotyls of Phaseolus do not react so vigorously to the 

 stimulus of light. If we cut young shoots of Sambucus nigra, 

 and, after stripping off their leaves, place them with their lower 

 ends in water, and then illuminate them from one side, we find 

 that they only slowly bend towards the light. Their heliotropic 

 irritability appears at all events to be insignificant, but further 

 information can only be obtained by searching investigation with 

 the clinostat, with elimination of geotropic nutations (see also 

 below). 



It is also very convenient, especially in demonstrations, to prove 

 the occurrence of heliotropic nutations by means of a so-called 

 heliotropic chamber. This consists of a box made of stout card- 

 board, about 16 cm. high, 20 cm. long, and 12 cm. broad (see 

 Fig. 158). The box is covered inside with dull black paper, and 



