68 
Peirce. — A Contribution to the 
if the contact be not long continued the effects, though plainly 
evident, are only temporary ; and that contact with an irritant 
object need not be permanent to produce permanent effects. 
But Cuscuta is by no means so sensitive as the most sensitive 
tendrils. Negative geotropism plays a much more important 
role than in the case of tendrils ; for repeated experiments 
show that the parasite will not twine about horizontal branches 
or rods, even though they be rough and thereby strongly 
irritant. If one complete turn be made about a vertical rod 
and the plant be evidently ready to make more, it will not 
continue to do so if the support be laid horizontally ; but on 
the other hand it will only partly, if at all, undo the turn 
which it has already made. Also, if two or more close turns 
be made around a vertical rod, the formation of haustoria will 
not be prevented or delayed by placing the rod horizontally. 
It is now evident that the short close turns of the Cuscuta 
about its host, or about any other support of suitable diameter, 
are induced by contact-irritation. It is also plain, from the 
observations of many authors, that haustoria are normally 
formed in nature only on such close turns. But is it necessary 
for the health of a branch that it form such close turns ? And 
are such close turns necessary for the formation of haustoria ? 
Both questions can be answered in the negative, as the two 
following experiments show. 
A strong horizontal branch of C. glomerata , springing from 
the axil of a bract borne on a stem just above a region where 
many strong haustoria had penetrated a very nutritious branch 
of Impatiens Sultani , was allowed to grow horizontally with- 
out contact with anything, until it had reached a length of 
fifteen centimetres or more ; then it was supported at a point 
far enough behind the tip to be no longer irritable. It con- 
tinued to grow horizontally, and supports were applied at 
suitable distances. Finally at the end of three weeks, having 
sent out numerous branches (which I cut off lest, in the 
crowded greenhouse, they should by accident come into con- 
tact with some other plant, and so vitiate the results of the 
experiment), the main branch, large, healthy, in normal con- 
