IOO 
Peirce , — A Contribution to the 
tinuation of these processes the swelling constantly increased 
in size, but more especially in height, and pressed the epider- 
mal cells which had already been for some time in contact 
with the host more and more strongly against it. From 
their nature and arrangement the peripheral tissues of this 
host could not resist or transfer the pressure ; they were 
forced to bend in and collapse ; and so the pressure became 
evident. A moment’s study of the much more delicate and 
yielding stem of Impatiens Balsamina will show how its peri- 
pheral tissues are affected by the pressure of the growing 
haustorium. Immediately underlying the single-layered epi- 
dermis which covers this essentially cylindrical stem, are five 
layers of collenchyma-cells which form a fairly strong and 
decidedly elastic ring about the deeper tissues. Within this 
ring are five or six layers of large rather thin-walled paren- 
chyma-cells which in turn enclose the more or less continuous 
ring of vascular bundles. There is an abundant central pith. 
We see that the stem is composed of four concentric rings 
enclosing a core of pith. These rings are not rigid, for they 
are composed, except the wood, of elastic and not thick- 
walled cells with larger or smaller air-spaces between them. 
Even the wood is composed of comparatively thin-walled 
ducts and fibres, and is so frequently traversed by medullary 
rays that this ring also is anything but rigid. Hence pressure 
exerted upon one part of the stem can be transferred to and 
divided among all parts, a slight change of form of the whole 
stem allowing the cells in the region first pressed upon to 
escape for a considerable time much if not all injury from 
this cause. Furthermore, the structure of the stem is such 
that horizontal pressure is not only distributed among the 
cells on the same plane, but among those above and below 
also, and so its effects are rendered still less evident. If it 
were not that the host is tightly embraced within the close 
spiral formed by the parasite, we could conceive of its being 
able, by sufficient change of form, to escape being penetrated 
by the haustorium which, after all, is able to exert only a 
limited amount of pressure ; but no stem strong enough to be 
