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 aérial 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 wzce 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 
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