90 
Peirce. — A Contribution to the 
effect, which, far from being opposed to contact-irritation, is 
in most instances supplementary to it, heliotropism and hydro- 
tropism play a very small part in the economy of these 
parasites. The two forces which affect the plant most 
strongly when it is in normal condition are geotropism, which 
is such an important factor in all climbing plants \ and contact- 
irritation, upon which all tendril-bearing plants depend to 
a greater or less extent for the accomplishment of the purpose 
for which the tendrils were designed. 
Since Cuscuta is an annual herb, it, like other such plants, 
passes through the purely vegetative stage with considerable 
rapidity. Plants of Cuscuta which are cultivated for experi- 
mental purposes in a greenhouse attain their maximum of 
vegetative development somewhat earlier than when cultivated 
out of doors ; but all the species (about ten in number) which 
have come under my notice, whether growing in a greenhouse 
or out of doors, were in full flower by the end of July. Up to 
a certain stage in the growth of a plant, all the buds which 
form in the axils of the scale-like bracts, the only traces of 
leaves which these parasites produce, develop into branches 
which in turn bear only vegetative buds. Presently some of 
the buds develop into flower-clusters. From this time on 
more and more buds develop into flowers, until finally few if 
any give rise to branches. Most of my plants of C. glomerata 
ceased to form new vegetative branches before the end of July, 
all their energy being devoted to flowering and setting seed. 
The plants of C. europaea , on the other hand, did form a few 
branches (very few in proportion to the size of the plants) 
until mid-October, but during the time of maximum flowering 
it too formed no new vegetative branches. I suspect that 
the greater hardiness of Chrysanthemum as compared with 
Impatiens has a not inconsiderable effect on its parasite, 
enabling it to vegetate longer than that on Impatiens. 
The flowers of the three species discussed in this paper are 
sessile and in dense clusters. Manifestly when the plant is in 
full flower, there being no new vegetative branches, it is no 
1 Wortmann, J., Theorie des Windens, Botanische Zeitung, 1886. 
