328 OHIO EXPERIMENT STATION: BULLETIN 214 
plants of the higher classes are wrapped about by a covering of skin 
or bark, and the food-filled juices are within; to feed upon any living 
host the parasite must gain access to the internal tissues of that 
host. Itso happens that there are minute openings or stomates 
(breathing pores) through the skin of leaves and of young green 
stems; these openings are as necessary as the stable door, and 
through them the thief may enter. (See Fig 4). Were these open- 
ings to become entirely closed the plant would languish, and remain- 
ing open, they constantly offer a way for the tender tip of the 
growing germ thread of a fungus to push its way through the plant 
covering and to luxuriate within the host upon the substance of the 
plant. 
Once within, the fungus thrives, rapidly multiplies its 
branches, and if in summer, commonly 
thrusts its fertile threads through 
some of these breathing pores to bear 
its spores outside where they may 
become more widely distributed than 
if remaining within the tissues of the 
host plant. Should, however, the 
Fig. 12. Haustoriaof the fungus of the 
grape downy mildew penetrating cells of 
grape stem. ‘The shaded portion shows 
the mycelium of the fungus growing be- 
tween the cells, sending haustoria, ¢ a, 
into the interior ofthe cells. (After Scrib- 
ner from Farlow). 
Note: In this figure the lower row of 
cells have the form of empty epidermal 
cells in which the fungus. would find little 
winter season be near, resting spores 
may be formed, or their formation be 
provided for within the leaves, or dis- 
eased parts, as in grape downy mil- 
dew, elm-leaf disease and in black-knot 
of plumandcherry. ‘Thus the cycle. 
of development continues indefinitely 
to subsist upon. Farlow’s original figure 
does not give these cells such form. unless some agency intervene to 
destroy the spores, to prevent their 
germination, or the parasite itself so exhaust the host plantas to 
destroy it entirely and the fungus perish for lack of suitable nidus. 
However, this rarely occurs, not perhaps, so often as men are guilty 
of killing the goose which lays the golden egg. Herein, we meet 
another fact, namely, that parasitic fungi of a given kind are limited 
toa particular host plant of a certain species, or toa small number 
of related plants, so that if a congenial host is lacking the fungus 
will not thrive. 
The fungus threads growing within any plant will not flourish 
if simply passing between the cells of the host; penetrating organs 
pierce the cell-walls and are able to absorb nutriment from the cell 
interior. (Fig.12). The diverse forms of sucking organs, and the 
peculiar structures of fungus threads in these situations would in 
themselves require much study and investigation to present them 
properly. We must further conceive that a fungus may often 
