Physiology of the Genus Cuscuta. 75 
marked difference between them and those adjacent is to be 
seen, as has just been briefly described as existing when the 
parasite is in similar contact with a host. That it is not the 
dryness of such rods of wood, glass, paper, or whatever the 
material may be, which stops the development may easily be 
shown. If water be made to flow gently and constantly over 
such rods, the epidermal cells differentiate little if any more, 
and the haustoria do not develop any more, than if the rods 
remain dry. If, after haustorial formation has been strongly 
induced by a wet or dry rod, the rod be withdrawn and the 
branch be enclosed in a moist chamber made of a piece of 
large glass tubing, sand being carefully poured into the tube 
and around the branch to a height just above the last turn, 
and wetted with distilled water, both ends of the tube being 
closed by corks, no more development of haustoria takes 
place than has already been described as occurring when the 
contact is only temporary. This experiment shows plainly 
that it is not the hardness of the rods which causes the epi- 
dermal cells to remain only slightly differentiated and the 
haustoria abortive, for the haustoria could easily penetrate 
between the comparatively loose particles of sand, and surely 
the sand would cause sufficient irritation, if contact were all 
that is necessary to cause a full development of these two 
structures. The sand used was first treated with strong 
hydrochloric acid to remove all calcium salts, washed on 
a filter with distilled water till the filtrate gave no acid 
reaction, then heated for a quarter of an hour over a Bunsen 
flame to carbonize all organic matters not decomposed by the 
acid, and finally cooled and poured into the glass tube, which 
had previously been washed with strong alcohol and then 
with distilled water. Of course the cork which is used to 
close the lower end of the glass tube must be cut in halves, 
and a groove, large enough to enclose the branch of Cuscuta 
without pressing it closely, be made between the two pieces. 
Thus we see that the continued contact of an easily pene- 
trable substance is not sufficient to induce full development 
of the haustoria and of the overlying epidermal cells ; nor are 
