254 BOTANY pakt i 



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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 scandens), in the hypocotyl of germinating 

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

 tendrils (chiefly in those with halptera or holdfasts), and in the stems 

 of some tendril -climbers. By means of their negative heliotropic 

 character, the organs for climbing and attachment turn from the light 

 towards their support, and are pressed firmly against it. 



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 steins of Tropaeolum majus, for example, exhibit positive heliotropic curvatures 

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

 retardation of their growth, they 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 assimilating, 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. The leaf-blades themselves, 

 and also the thalli of the Cryptogams, are capable of carrying on 

 transversely heliotropic movements, while the movements of the 

 growing portions of leaf-stalks seem to be influenced by their leaf- 

 blades. 



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 Lacluca Scariola and the North American Silphium lacinia- 

 twin necessarily take the direction of north and south, and so are often referred to 

 as compass plants. (As regards the vertical position of phyllodes, in connection 

 with which may be mentioned the vertically-placed leaves of many Myrtaceae and 

 Proteaceae, see p. 195.) 



The heliotropic character of organs may change through the activity 

 of external influences, and also at different stages of their development 

 and growth ; just as in the case of the heliotactic swarm-spores, the 

 higher plants in ordinary light may be positively, and in very intense 

 illumination negatively heliotropic. The youngest portion of the shoots 

 of Ivy and Tropaeolum are positively heliotropic, while the lower and 

 older portions turn away from the light. The flower-stalks of Linaria 

 cymbalaria are at first positively heliotropic. After pollination, how- 

 ever, they become negatively heliotropic, and as they elongate they 

 push their fruits into the crevices of the walls and rocks on which 

 the plant grows, and thus assure the lodgment of the seeds and the 

 possibility of their future germination. 



