274 Ward . — On Relations between Host and Parasite 
It is easy to see a tuft of paper-pulp, e. g., scraped filter- 
paper, with which spores have been swabbed up, and which 
has been placed in a young leaf-axil, lifted up bodily by the 
growth of the basal parts ; and next day, instead of the germ- 
tubes having entered the tissues of both young leaves with 
which the contaminated swab was in contact, the whole mass 
may have been lifted up on one leaf, or fhrown off bodily on 
to the ground. 
If such a ‘ swab ’ is placed on the apex of the growing leaf, on 
the contrary, it is lifted up as the tip rises and infection occurs. 
This ready infection of the tips, the oldest part of the grass 
leaf, seems to explain why pustules ordinarily appear first at 
the apex of the leaves. The latter are most easily infected 
there because the 6 tip-drops,’ emerging from the water-stomata 
under the action of root-pressure, catch spores readily, and 
promote their germination just on that part of the leaf where 
the stomata are most mature ; and this would apply parti- 
cularly to wind-blown spores which lodge in or near the leaf- 
axils in the tufts of grass. 
In sowing the Uredospores I have tried various plans. 
The simplest is to place the dry spores direct from the 
pustules on the surface of the leaf by means of the tip of 
the blade of a knife or a flattened platinum wire. Another 
is to brush them on with a camel-hair pencil. They may also 
be sown in water, and a drop of the latter then placed on the 
leaf by a fine pipette or a wire, &c., but some difficulty is 
usually met with owing to the tendency of such drops to roll 
off the slightly waxy or hairy leaf-surface. I found it at first 
by no means easy to attach such drops to the leaf-surface, 
especially as the latter is usually vertical. 
An interesting proof that this is owing to the difficulty of 
wetting the leaf is afforded by the following observation. 
If one takes a slender capillary pipette and blows down it so 
as to direct the fine stream of moisture-laden air on to a spot 
on the leaf-surface, the latter will readily take the drop at 
that spot and hold it. I take it this is owing to the fact that 
moisture has condensed on the cuticle in extremely fine 
