72 
Peirce . — A Contribution to the 
of haustoria deep in the cortex, which push out the overlying 
cortical and epidermal cells. No change in the form of the 
epidermal cells takes place, though they must multiply in 
order to maintain an unbroken covering for the stem. After 
the swellings have ceased to grow, sections of them show that 
the haustoria attained a greater or less development in pro- 
portion as the part of the stem was in longer or briefer contact 
with the host ; but in no case where the contact was not 
permanent was there a complete development. 
That haustoria never begin to form without contact has 
been repeatedly shown by placing branches which are ready 
to form haustoria, whenever the conditions become favourable, 
in various nutrient solutions. Haustoria never form in such 
circumstances. If, however, a rod of suitable size be brought 
into contact with the submerged branch, feeble twining will 
take place and sometimes haustoria will be begun ; but the 
process of twining and, as a consequence, the formation of 
haustoria are hindered by the fluid. The hindrance is doubt- 
less mainly of one sort, that which reduces the irritating 
effect of the rod. This result resembles in kind, though not 
in degree, those obtained by similar experiments with sensi- 
tive tendrils 1 ; but owing to their greater sensitiveness to the 
contact of solid bodies, the fluid, be it water or anything else 
not poisonous, does not so greatly reduce the amount of 
twining. Evidently contact is absolutely necessary for the 
beginning of haustoria, and it is plain too that the formation 
of haustoria is an induced process, just as the winding of 
tendrils and the formation of short, close turns are induced 
processes. 
The swellings just mentioned as being produced by the 
outward pressure of the growing, but soon aborting, haustoria, 
differ in shape from those formed when the parasite remains 
at those points in contact with a host-plant. These swellings, 
the results of the arrested growth of haustoria, are conical 
in form and are covered by flat and somewhat elongated 
1 Pfeffer, W., Zur Kenntniss der Kontaktreize, Untersuch. aus dem Bot. Institut 
zu Tubingen, Bd. I, p, 483 et seq. 
