88 
Peirce. — A Contribution to the 
direction desired. The stem is positively heliotropic. Expo- 
sure to light from one direction for only six hours will cause 
the tips to point almost straight toward the source of light. 
If, however, the plant be simply laid horizontal, and then be 
illuminated from the side only, that is, so that the effect of 
heliotropism would be to cause the plant to grow in the hori- 
zontal plane, it will be noticed that the tips point almost 
vertically upwards, plainly showing how much stronger the 
effect of geotropism is than that of heliotropism, so much 
stronger indeed that most authors have hitherto agreed in 
saying that Cuscuta is not heliotropic at all. That it is some- 
what heliotropic is proved by the marked effect of light on it 
when geotropism is excluded. 
Light has little effect on the formation of haustoria, and 
none on the close twining of the plant which usually 
precedes their formation. If a branch of C. glomerata in 
contact with a wooden rod be enclosed in a dark chamber, 
the branch will continue to twine as rapidly and for as long 
a time as usual, and the formation of haustoria will be begun 
within the usual length of time after firm contact has been 
established. If anything, the branch will twine rather faster 
in the dark than in the light, and its haustoria will also become 
evident somewhat earlier ; for this plant is no exception to 
the general rule that growth is faster at night (that is, in 
darkness) than during the day. Plainly the absence of light 
is no hindrance to the formation of haustoria ; but is light 
a hindrance ? They arise within the concave half of the close 
spiral formed about a host, and this is the darker half, since 
more light necessarily falls on the outer and convex surface 
of the coils. If we cause the formation of haustoria between 
two leaves held together face to face by two plates of glass, as 
already described on p. 69, we can determine the effect of 
light on haustorial formation by illuminating the two sides 
equally and unequally. When the two leaves are equally 
illuminated, the numbers of haustoria on the two sides of the 
Cuscuta are approximately equal. When one side of the stem 
receives more light than the other, the haustoria are somewhat 
