SECT. II 



PHYSIOLOGY 



305 



(p. 262). Other pliototropic phenomena were found to be at variance with this 

 explanation of phototropism. Unicellular perfectly transparent fungal hyphse are 

 also subject to positive curvature, although in this instance there can be no shaded 

 side ; on the contrary, the side of a hypha turned away from the light is especially 

 illuminated on account of the refraction of the light rays. The fact, too, that 

 negative curvatures also take place renders it evident that phototropism cannot be 

 due to one-sided etiolation ; for in negative phototi'opism the side most directly 

 illuminated is the one that grows more rapidly, although the retarding eft'ect of 

 light on the normal groAvth in length of negatively pliototropic organs is equally 

 operative (roots, rhizomorpha). 



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

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

 but the direction in which the most intense light rays enter the organs. 

 Light acts as a motory stiimulus when it penetrates 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 intensity of the illumination the plant is kept in constant 

 rotation, around a vertical axis, by means of clock-work, the photo- 

 tropic stimuli acting on the different sides neutralise one another and 

 no curvature takes place. This apparatus is known as a KLINOSTAT. 



The pliototropic 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, although the red light is of greater importance for its carbon 

 assimilation. 



Sensibility to photo tropic influences is prevalent throughout the vegetable 

 kingdom. Even organs like many roots, which are never under ordinary 

 circum.stances exposed to the light, often exhibit pliototropic irritability. Positive 

 phototropism is the rule with aerial vegetative axes. Negative phototropism is 

 much less frequent ; it is observed in aerial roots, and sometimes also in climbing 

 roots (Ivy, Ficus stipulala, Begonia scandens), in the hypocotyl of germinating 

 Mistletoe, in many, but not all, earth roots {Smapis, Hclianthus), in tendrils 

 (chiefly in those with attaching discs), and in the stems of some climbers. By 

 means of their negative heliotropie character, the organs for climbing and attach- 

 ment, and the primary root of the Mistletoe, turn from the light towards, and are 

 pressed firmly against, their darker supports. 



Negative pliototropic curvatures are occasionally produced, not in the region 

 of most rapid growth, but in the older and more slowly growing portions of the 

 stem. The stems of Tropacolum majus, for example, exhibit positive pliototropic 

 curvatures in the region of their greatest elongation, but lower down the stems, 

 where their growth is less rapid, become negatively pliototropic. 



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 ;hese organs transverse phototropism, in 

 conformity with its great utility for assimilation, predominates over 



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