29 
Lying in the soil near the seedling, these oospores germinate (in the case 
of P. Cactorum at least) by sending forth a germtube at the tip of which is 
produced one or more conidia. Study Cornell Bul. 363, fig. 18. copy 
one or more of the drawings. 
These conidia germinate either by swarmspore-formation or by germ- 
tubes and infect the tender tissues of the seedlings. (See Cornell Bul. 363, 
fig. 16-17.) Conidial germination will be studied later. 
The mycelium developed from the primary inoculum spreads rapidly 
in the succulent tissues of the seedlings, secreting enzymes which enable 
it to penetrate the cellulose walls, and toxines which kill the protoplasm. 
Study thin longisections (freehand or prepared) through diseased stems. 
OBSERVE :-— 
6. The relation of the mycelium to the host-cells; inter- or intra- 
cellular? Are haustoria produced? 
7. The structure of the mycelium; densely granular, branched, 
non-septate, often knobbed and swollen. The form and structure may 
often be better made out in mounts from pure culture. 
Make and study mounts from cultures provided. OBSERVE :— 
8. That frequently there is a streaming of the cytoplasm in the 
long non-septate cells. 
_ DRAW, showing structure of the mycelium and its relation to the host- 
tissues. 
From this mycelium is sent forth from the diseased tissues short fruit- 
ing branches,—the conidiophores, on which are developed conidia just 
like those produced from the oospore. Make mounts from diseased 
stems or from pure cultures showing conidia. Study and OBSERVE :— 
9. Their peculiar form, apical papillae and coarsely granular 
contents. | 
10. Their attachment to the conidiophore at their larger 
end. DRaw showing conidiophores with conidia. These conidia function 
as the inoculum for the secondary cycles. In P. de Baryanum it is held 
that mycelium spreading through the soil from seedling to seedling usually 
functions to initiate the secondary infections. This may also be true 
at times for P. Cactorum. 
Saprogenesis. The mycelium in the dead tissues continues to 
grow and produce conidia so long as the food supply is not exhausted. 
As the nutrients begin to fail there develops on the mycelium within the 
tissues the sexual bodies, which eventually mature into oospores. Tease 
apart in a drop of water bits of diseased stems which contain forming 
oospores or use mounts from pure cultures provided. OBSERVE >— 
11. The large globose bodies,—the oogonia, and the smaller 
closely appressed antheridia. 
12. The manner in which they are borne on the mycelium and 
their positional relation to each other. 
13. Their thin, hyalin walls and granular contents. 
14. The aggregation in some, of the denser protoplasm in the 
center,—the oosphere, and the surrounding layer of hyalin protoplasm,— 
the periplasm. 
15. The impregnation-tube penetrating from the antheridium, 
through the oogonium wall and periplasm to the oosphere. What is the 
function of this tube? 
