240 SECTIONAL ADDRESSES. 
resistant cuticle, it is evident that it must have some point d’appui against 
which the force can be expected. There must clearly be some adhesion 
of the germ-tube to the substratum or else the development of an outgrowth 
from the germ-tube will result, not in penetration, but merely in the forcing 
the tube away from the surface. It was originally suggested" that the gela- 
tinous sheath which can be demonstrated round the germ-tubes of Botrytis 
cinerea, and of some other parasitic fungi, is the main factor in the close 
attachment of the tube to the substratum. Further consideration, 
however, suggests that the essential preliminary to penetration is the close 
adhesion of the tip of the germ-tube to the surface to be penetrated. This 
close adhesion to the surface to be penetrated is a constant feature of 
epidermal infection, whether we are dealing with Botrytis cinerea, Sclerotinia 
Libertiana, Puccinia graminis (sporidia), or the case of Colletotrichum where 
the tip of the germ-tube becomes converted into a dark-coloured, thick- 
walled appressorium from which the infection-tube grows out later. In 
this adhesion the two gelatinous sheaths may play some part, but when 
one considers the ‘ microscopic’ closeness of the contact it would seem 
clear that molecular forces must be at work, so that once they are brought 
into such close relationship the two surfaces would necessarily adhere.!? 
This sheath may, however, be of use in preventing the germ-tube from 
being easily washed off the surface on which it is growing and also in 
giving the germ-tube the attachment necessary if the tip is to be pressed 
firmly against the surface of the leaf or other organ. Once, however, 
the two surfaces are pressed together they should adhere in thé manner 
indicated. 
It will be noted that the adhering surface from which the infection 
tube grows out is in general large compared with the cross section of the 
actual peg-like infection hypha which bores through the cuticle (fig. 2). 
This hypha is very small, and in the case of sporidial infection in Puccima 
and infection by Synchytrium endobioticum it is of extreme tenuity, so that 
in the epidermal wall itself it can only just be observed. The absolute 
pressure required to push such a minute infection hypha through the wall 
would be very small, and the forces of adhesion which hold the tip of the 
germ-tube (or the body of the zoospore in S. endobioticum) to the surface 
of the host cell would seem to be more than suflicient to resist the back 
pressure resulting from the outgrowth of the infection tube. The processes 
concerned in the development of this outgrowth are probably very similar 
to those concerned with the development of a lateral branch on a hypha. 
If one assumes that the cell-wall of the germ-tube becomes softened over 
the appropriate area, then the osmotic pressure of the contents of the germ- 
tube should be more than sufficient to overcome the resistance of the cuticle 
and the sub-cuticular layers of the cell-wall. It is interesting to note that 
Hawkins and Harvey conclude that mechanical puncture is the method by 
which Pythium debaryanum passes after entry through the ordinary cell- 
walls of the potato tuber, and that resistance of the tuber cells to 
11 Blackman and Welsford ; ‘ Infection by Botrytis cinerea.’ Annals of Botany, xxx., 
389, 1916. 
12 This suggestion that such molecular forces come into play was originally put 
forward in a discussion by Dr. A. L. Balls. 
13],, A. Hawkins and R. B. Harvey : ‘ Physiological Study of the Parasitism of 
Pythium debaryanum Hesse, on the Potato Tuber.’ J. Agric. Res., XVIII., 275, 1919. 
Le aa th i 
