108 Peirce . — A Contribution to the 
the cells in which it is stored, or whether by a less violent 
process they are enabled to act upon it. One can demonstrate 
this by means of a slender stick of elder-pith impregnated 
with a nutrient solution, as was used in determining the condi- 
tions for haustorial formation (see p. 76). Let such a stick 
of pith be put in contact with the irritable part of a branch of 
Cuscuta enclosed in a moist chamber of large glass-tubing, 
quickly closing the chamber. After a few days, the Cuscuta 
having made many turns about the stick and bearing haustoria 
in several stages of development, one may sever the branch 
from the parent and remove it, still in contact with the pith, 
from the chamber. Sections through haustorial swellings 
which seem firmly fixed in some way to the pith, will demon- 
strate that the papillate epidermal cells are as large and 
healthy as normal, and that some of them have penetrated 
the cells of the pith to a greater or less depth, some even into 
the third row from the surface. These pith-cells seem little 
compressed, certainly not collapsed or ruptured. Clearing 
thin sections by means of equal parts of glycerine and water, 
and examining them under a high power of the microscope, 
we see that the epidermal cells have entered the cavities of 
the pith-cells through holes in the walls. These holes corre- 
spond with considerable accuracy to the shape and size of the 
cell or cells passing through them (Fig. 7). By staining with 
chlor-iodide of zinc and washing away the excess by distilled 
water before clearing in the mixture of glycerine and water, 
the result will be made still more evident ; for the difference 
in composition of the cell-walls of the pith and of the Cuscuta , 
with the resulting differences in the shades of blue produced 
by the reagent, and the presence of protoplasm and nucleus 
(stained yellow or brown) in the cells of the parasite and the 
absence of these structures in the pith-cells, enables one to see 
very clearly that not by pressure alone could the epidermal 
cells have thus made their entrance. These epidermal cells, 
collectively forming the ‘ pre-haustorium/ must have secreted 
an enzyme which at least has softened and, since one can see 
no broken pieces of cell-wall turned back or pushed to one 
