INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



279 



across the sky, and in the evening they all face the west. They close about 

 sunset and then become erect on their stalks, remaining so until morning, when 

 movement begins anew. This movement can be stopped by very intense light. 

 In Fig. 130 are shown closed and open flower-heads of Hieracium, a plant 

 closely related to Tragopogon and showing the same responses. 



Phototropic bending occurs also in non-green plants — in moulds, for example. 

 If fresh horse dung is placed in a closed chamber with a small glass window, a 

 dense growth of Pilobolus soon develops and the sporangiophores all bend to- 

 ward the window. The sporangia, containing the ripe spores, are thrown with 

 considerable force, well-aimed at the glass window, to which they adhere (Fig. 



i3i)- x 



Negative photropism is not 



very common, but occurs with 1 



many tendrils and aerial roots. 



Weisner 2 studied the aerial roots 



1\ 



Fig. 130. Fig. 131. 



Fig. 130. — Flower of Hieracium pilosella. A, open, as by day; B, closed, as by night. 



Fig. 131. — Diagram showing phototropic response of Pilobolus. The culture is in a 

 chamber and receives light only through small window at left. Spore-masses are discharged 

 toward the window. 



of sixty-one different plant forms and found that the negative phototropic re- 

 sponse was very marked in twenty-seven species and was not so marked in 

 twenty-four species, while six species showed but little sensitiveness to'light 

 and the remaining four were not sensitive at all. This phenomenon does not 

 occur commonly in ordinary subterranean roots, but if mustard seedlings 

 (Sinapis alba) are grown in water-culture it is easy to demonstrate both posi- 

 tive phototropism of the shoots and negative phototropism of the roots. 



Phototropic bending results from unequal growth on the two sides of the 

 organ in which this bending occurs, and the response takes place only in the 

 enlarging region. The degree of bending, or its rate, depends upon light 

 intensity. Light of medium intensity produces the most pronounced bending, 

 and the response is less marked both with higher and with lower intensities. The 

 phototropic response is slight when the light intensity is low, increases to a 

 maximum with medium light intensities, and becomes less when the light in- 



1 For an excellent study of the light reaction of Pilobolus, see: Parr, Rosalie, The response of 

 Pilobolus to light. Ann. bot. 32: 177-205. 1018. 

 - [Wiesner, 1879, 1882. [See note 2, p. 275.] 



