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Pathogenesis. There are two quite different sources of primary 
inoculum; one the dead overwintered leaves, diseased the previous season; 
and the first leaves put forth from systemically invaded plants. 
(a) Inoculum from overwintered leaves. Overwintered hepatica leaves, 
showing last year lesions, have been cleared by special treatment and 
placed in alcohol in vials. Holding one of these to the light, examine with 
the hand-lens and OBSERVE :—- 
6. The great number of minute globose dark-colored oospores 
imbedded in the tissues. SKETCH a portion of the leaf to show these. 
These germinate in situ, sending forth from each oospore a slender 
conidiophore bearing one or more conidia. Oospore-germination is not 
readily obtained. It probably does not differ materially from that des- 
cribed by Gregory for the oospores of Plasmopara Viticola in grape leaves. 
(Study Gregory’s article, separate from Phytopath. 2:237, fig. 2). copy 
and label to explain the use of these figures. 
Borne by wind or splashed by rain-drops, these conidia reach the 
developing leaves of their respective hosts and initiate the primary infect- 
tions. 
(b) Inoculum from systemically invaded plants. The pathogene 
winters as mycelium in the rootstocks of such plants. As the first leaves 
develop the mycelium develops with them and shortly after the leaves 
unfold, sends forth numberless conidiophores through the stomata on the 
under surface. These conidiophores develop conidia (primary inoculum) 
in every respect like those produced from the oospores. Mount some 
of the down from the under surface of a systemically invaded leaf. 
OBSERVE —— 
7. The numerous globose or egg-shaped conidia; their thin walls 
and densely granular contents. praw. Carried by the slightest breeze, 
these conidia fall upon young leaves of their host-plants and, if moisture 
is present, germinate and infect the plants. 
Conidia, whether produced from oospores or from mycelium from 
within the living leaf, germinate in the same way. If they are conidia of 
Plasmopara they form swarmspores. If they are conidia of Peronospora 
they germinate by a germtube. If viable conidia of either or both are 
available, study germination and pRaw to show the structures produced 
or copy drawings provided by the instructor. 
Mycelium is rapidly developed from the germtubes of conidia or swarm- 
spores and spreads through the tissues. - 
. Make thin sections of diseased leaves of anemone or hepatica (if dry 
leaves, mount sections in potassium hydroxide) and study carefully to 
LOCATE -— ae 
8. The mycelium. Is it intercellular or intracellular? Are 
haustoria present; what form?’ 
9. Mycelial branches projecting forth through the stomata; 
how many through each stoma? The form of these conidiophores is 
best studied in material scraped from the under surface of alesion. Scrape 
a bit of the white felt from the under surface of one of these lesions on 
anemone or hepatica leaves, mount in potassium hydroxide and examine 
with the low-power. OBSERVE:— 
10. The short, rather stout, scarcely branched conidiophores. 
Do not confuse them with the long pointed thick-walled leaf-hairs. It 
is from the unusual shortness of the conidiophores that this fungus gets 
