104 Peirce. — A Contribution to the 
contact with a glass rod whose average diameter is known; 
and another branch, which is not in the irritable stage, be 
brought into such a relation with another rod of the same 
diameter that it will make steep turns about it. After both 
branches have twined about the supports for from thirty-six 
to forty-eight hours, remove the rods with as little disturbance 
of the coils as possible. It will at once be noticed that the 
diameter of both coils decreases, and that the number of turns 
increases proportionally. After waiting from twelve to 
twenty-four hours, the contraction will have reached its 
maximum and have become permanent. By measuring now 
the average diameters of the two imaginary cylinders enclosed 
by the gradual and the steep spirals respectively, by the close 
turns formed in consequence of irritation and by the loose 
turns formed without irritation (that is, as a climbing plant 
would form them), we find that the diameter of the former is 
from one-half to three-quarters the diameter of the rod first 
employed, whereas the diameter of the latter is never less than 
three-quarters the diameter of the rod. Evidently the steep 
spiral, formed only for supporting the plant as it ascends, 
exercises little if any more than enough pressure on the 
support to keep the plant from slipping down by its own 
weight ; whilst the close spiral, formed in consequence of 
irritation for the purpose of forming an intimate and enduring 
contact and for enabling the haustoria the more readily to 
penetrate the host, exercises very considerable pressure. As 
shown before, the pressure exercised by the closely coiled 
stem is still more increased by the formation and growth of 
the haustoria. 
But the force brought to bear on the host by the parasite, 
whether through the gradual or the steep spirals, is not merely 
one of compression ; for by its circumnutation the parasite 
exercises a force which, unfortunately, has not yet been 
determined. We must therefore recognize two sorts of force 
which are applied to the host ; a deflective, exercised by 
circumnutation, and a compressive, exercised by the coils and 
the haustoria growing in and from them. Doubtless the sum 
