350 



BOTANY 



PART I 



curvature even in trees where growth in length had ceased. It was formerly held 

 that the increased growth of the shaded side in positive phototropism was produced 

 by the beginning of etiolation, and that the diminished growth on the illuminated 

 side was due to the retarding effect which light exerts upon growth in length 

 (p. 289). This view has for some time been abandoned for good reasons ; it 

 cannot be maintained even in the modified form in which it has been recently 

 stated by BLAAUW ( 106 ). The fact that in many cases the curvature is far removed 

 from the region stimulated by light (p. 351) is especially opposed to this explanation. 



It is evident from these considerations that it is not the difference 

 in the intensity of the light which causes the heliotropic curvatures, 



but the direction in which 

 the most intense rays of 

 light enter the organs. 

 LIGHT ACTS AS A MOTORY 

 STIMULUS WHEN IT PENE- 

 TRATES AN ORGAN IN ANY 

 OTHER DIRECTION THAN 

 THAT WHICH CORRESPONDS 

 WITH THE POSITION OF 

 HELIOTROPIC EQUILIBRIUM. 

 Only one-sided illumination 

 can thus cause curvature in 

 a plant. If, without altering 

 the direction or the inten- 

 sity of the illumination, the 

 plant is kept in constant 

 rotation, around a vertical 

 axis, by means of clock- 



FIG. 284. Pilobolus crystallinus (P), abjecting its sporangia WO rk, the phototropic 

 towards the light. G, Sheet of glass ; B, opaque case , i , . f^ 



with a circular opening at F; M, vessel containing Stimuli acting on 



horse-dung. (Of. description in text. After NOI.L.) different sides neutralise 



one another and no cur- 

 vature takes place. This apparatus is known as a KLINOSTAT. 



The phototropic curvatures are most strongly produced, just as in the case of 

 the heliotactic movements of freely moving swarm-spores, by the blue and violet 

 rays, while red and yellow light exerts only a much slighter influence. When a 

 plant receives on one side red light, and on the other side blue light, it turns 

 towards the latter, even when the red light is of greater intensity. 



TRANSVERSE PHOTOTROPISM is confined almost solely to leaves and 

 leaf-like assimilatory organs, such as Fern prothallia and the thalli of 

 Liverworts and Algae. In these organs transverse phototropism, in 

 conformity with its great utility for assimilation, predominates over 

 all other motory stimuli. Such organs become placed at right angles 

 to the brightest rays of light to which they are exposed during their 

 development ; in this process torsions of the leaves or internodes 

 are combined with the simple curvatures. 



