Tropisms and Instincts as Adaptations 401 



see that this bending of the stem toward the light and the 

 turning of the leaves to face the light are only parts of the 

 general relation of the whole plant toward the light. 



Negative heliotropism is much less frequent in plants. It 

 has been observed in aerial roots, in many roots that are ordi- 

 narily buried in the ground, in anchoring tendrils that serve 

 as holdfasts, and even in the stems of certain climbers. In 

 all of these cases, and more especially in the case of the 

 climbers, the reaction is obviously of advantage to the plant ; 

 and it is significant to find, in plants that climb by tendrils 

 carrying adhering disks, that there is a reversal of the ordi- 

 nary heliotropism shown by homologous organs in other 

 plants. There is an obvious adaptation in the behavior of 

 the tendril, since its growth away from the more illuminated 

 side is just the sort of reaction that is likely to bring it into 

 contact with a solid body. 



In this connection it is important to observe that these 

 reactions to light are perfectly definite, being either positive 

 or negative under given conditions, and therefore there is at 

 present nothing to indicate that there has been a gradual 

 transformation from positive to negative, or vice versa. It 

 seems to me much more probable that when the structural 

 change took place, that converted the plant into a climber, 

 there appeared a new heliotropic response associated with the 

 other change. In other words, both appeared together in the 

 new organ, and neither was gradually acquired by picking 

 out fluctuating variations. 



The leaves of plants also show a sort of transverse heliotropic 

 response. It has been found, for example, that the leaves of 

 Malva will turn completely over if illuminated by a mirror 

 from below. A curious case of change of heliotropism is found 

 in the flower stalks of Linaria. They are at first positively 

 heliotropic, but after the flower has been fertilized the stalk be- 

 comes negatively heliotropic. As the stalks continue to grow 

 longer, they push the fruits into the crevices of the rocks on 



2 D 



