Physiology of the Genus Cuscuta . 89 
more numerous on the less brightly-lighted side. That there 
would be a great difference we ought not to expect, for the 
difference in the illumination of the inner and outer sides of 
a close-coiled spiral is also not great. 
We thus see that, despite the general opinion to the con- 
trary, Cuscuta is sensitive to light to a slight extent. Whether 
it is more sensitive to light (that is, more heliotropic) when it 
is green I cannot say ; nor would a positive answer to this 
question be of great value in any way, for it is quite evident 
from what I have said that the green colour is formed only 
when the Cuscuta is under unfavourable conditions of nourish- 
ment. It is well known that other plants almost or entirely 
devoid of chlorophyll are sensitive to light and are positively 
heliotropic, and hence it can by no means be inferred that the 
comparative insensibility of this plant is due to its lack of 
chlorophyll. It is evident that great sensitiveness to light 
might frequently interfere with and counterbalance the effect 
of contact-irritation, and so render the plant unable to secure 
the necessary food. We must associate this comparative 
insensibility to light with its mode of attacking and spreading 
itself over a host, not with the absence of chlorophyll, which 
is merely a result of the plant’s securing an abundance 
of already elaborated food. The lack of chlorophyll is 
a degeneration in consequence of parasitism (as seen in 
many other parasites) ; the neutrality of the plant to light 
is one of the minor aids to its accomplishing the necessary 
close windings about a hos 1 :. 
That these plants, especially seedlings, are strongly 
influenced by moisture I have already pointed out ; but that 
their stems are positively hydrotropic I have been quite 
unable to see. If they were strongly hydrotropic they would 
in this way at times defeat their own ends ; for it is possible 
to conceive that contact-irritation and hydrotropic attraction, 
working on two sides of the plant and drawing it in opposite 
directions, might neutralize each other. But hydrotropism, if 
at all affecting the plant as a positive force, does so only to 
a slight degree. When geotropism is still exercising its strong 
