PHYSIOLOGY 277 



light is especially illuminated on account of the refraction of the light rays. The 

 fact, too, that negative heliotropic curvatures also take place renders it evident that 

 heliotropism cannot be due to one-sided etiolation ; for in negative heliotropism 

 the side most directly illuminated is the one that grows more rapidly, although 

 tin- retarding effect of light on the normal growth in length of negatively heliotropic 

 organs is equally operative (roots, rhizomorpha). 



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 light rays enter the organs. 

 LIGHT ACTS AS A MOTORY STIMULUS WHEN IT PENETRATES AN ORGAN 

 IN ANY OTHER DIRECTION THAN THAT WHICH CORRESPONDS WITH THE 

 POSITION OF HELIOTROPIC EQUILIBRIUM. 



The heliotropic 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 an extremely slight influence, or none 

 at all. It is due to the fact that the red-yellow and blue-violet rays are always 

 present together in daylight, that the heliotropism of the leaves is of advantage to 

 their assimilatory activity. Intermittent illumination has a more powerful effect 

 on the plant than light of uniform intensity. 



Sensibility to heliotropic influences is prevalent throughout the vegetable 

 kingdom. Even organs like the roots of trees, which are never under ordinary 

 circumstances exposed to the light, often exhibit heliotropic irritability. Positive 

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

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

 roots (Ivy, Ficus stipulata, Begonia scandcns), in the hypocotyl of germinating 

 Mistletoe, in many, but not all, earth roots (Sinapis, ffelianfJnts), in tendrils 

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

 of their negative heliotropic character, the organs for climbing and attachment turn 

 from the light towards, and are pressed firmly against, their support. 



Negative heliotropic 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 Tropaeoliimmajus, for example, exhibit positive heliotropic curvatures 

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

 growth is less rapid, become negatively heliotropic. 



TRANSVERSE HELIOTROPISM 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 heliotropism, in 

 conformity with its great utility for assimilation, predominates over 

 all other motory stimuli. Thus it is possible to cause the leaf-blades 

 of a Malva or a Tropaeolum to turn completely over by illuminating 

 their under surfaces by means of a mirror. 



In too bright light the transverse position of the leaves becomes changed to a 

 position more or less in a line with the direction of the more intense light rays. 

 In assuming a more perpendicular position to avoid the direct rays of the mid-day 

 sun, the leaf-blades of Lactuca Scariolu and the North American Silphium h/r /'///'>/- 

 turn necessarily take, according to STAHL, the direction of north and south, and so 



