1 1 6 Peirce.— A Contribution to the 
and geotropism. The latter are induced by contact-irritation, 
which causes a modification of the manner and an acceleration 
in the speed of coiling. 
Haustoria are ordinarily formed only upon the concave 
surfaces of the close coils. They also are the result of irrita- 
tion, their formation being induced by contact of sufficient 
duration. Their development depends upon both contact 
and nourishment ; without the one or the other only partial 
development takes place. The formation and perfect develop- 
ment of haustoria may take place on one as well as on another 
side of the stem, or on opposite sides at one and the same 
time, it being equally irritable on all sides, provided only the 
necessary contact and nourishment be supplied. They appear 
almost exclusively on the concave side of the close curves 
only, because there the contact-irritation is greatest ; they 
develop best there because both contact and nourishment 
are on that side. Each haustorium is formed as a result of 
contact with an irritant object over the place of its origin. 
Innocuous liquids and wet gelatine are not irritant objects. 
The periodic irritability of Cuscuta can be temporarily 
destroyed by revolving the plant horizontally around its long 
axis; that is, by neutralizing the effects of geotropism. In 
ordinary conditions geotropism is stronger than irritability, 
for the plant will not twine about horizontal supports ; though, 
when a once vertical support about which a branch has closely 
twined is laid horizontally, the formation and development of 
haustoria already induced go on unhindered. In ordinary 
conditions these plants are not markedly heliotropic or hydro- 
tropic ; but when the effects of geotropism are neutralized by 
horizontal revolution on the clinostat, the plants become more 
sensitive to light and moisture. The comparative insensitive- 
ness to light is not due to the absence of chlorophyll, nor is 
chlorophyll always absent. It is formed whenever for any 
reason the plant is insufficiently nourished, and the amount 
formed (that is, the intensity of the green colour) may be used 
as an index of the amount of organic food which it is receiving. 
These parasites can attack successfully only those plants 
