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thecium. These constitute the primary inoculum. praw to show the 
form and structure of these ascospores. 
When the ascospores are mature and the perithecium is thoroughly 
wetted, it cracks open and the ascospores are forcibly discharged. Borne 
by the wind, they fall upon the growing leaves of the host and germinate. 
If viable ascospores are available, study spore-germination as seen in the 
slides provided. Draw, or copy illustrations provided by the instructor. 
As soon as this germtube has developed a food-relation with the host 
by means of haustoria in the epidermal cells, a mycelium begins to develop, 
branching and spreading in all directions over the leaf-surface. Examine, 
under the binocula rmicroscope, one of the white mycelial mats on the 
diseased green leaves (fresh or preserved) provided and OBSERVE :— 
6. The tangle of silver-white hyphae with long spreading branches 
about the margin of the lesion. : 
7. The numerous erect chains of conidia borne on short conidio- 
phores. Many of these conidia have fallen off and give the mealy appear- 
ance to the mildew-spots. 
Scrape the mycelial mass from the surface of the leaf. Mount in water, 
cover and EXAMINE :— 
8. Conidia; large ellipsoidal, flattened slightly at the ends; 
their thin hyaline walls and densely granular protoplasm. 
9. Conidiophores; short, with a swollen base just above the 
point of attachment to the mycelium. Try to find a conidiophore with 
several conidia still attached. 
10. The mycelium; very crooked and much broken in scraping 
from the leaf; septate or nonseptate? 
As the mycelium spreads over the surface of the leaf it sends minute 
branches through the outer cell-wall of the epidermal cells. This branch 
enlarges and branches within the cell to form the finger-like haustoria. 
Study these in the slides provided or from the drawings by Smith, Bot. 
Gaz. 29, pl. XI and XII. 
Make a composite DRAWING to show a cross-section of the epidermal 
cells of the host with haustoria, mycelium, conidiophores and conidia in 
normal relation to each other. 
These conidia break off at the top of the chain as fast as matured and, 
scattered by the wind, initiate secondary cycles. 
Conidia continue to be produced for a time by the mycelium on the 
primary lesions. As the primary lesions are largely on the young or 
seedling-leaves of the host, the mycelium probably perishes along with the 
young leaf before the sexual fruit-bodies can be developed. These appear 
later on the mycelium of the secondary cycles. There is, therefore, no 
saprogenic phase in the primary cycles. ; 
The Secondary Cycles are initiated by conidia from the primary 
cycles. 
Pathogenesis. Thefungusexhibitsthe same conidial structures in the 
secondary cycles as those just studied. As the host-tissues begin to mature, 
conidial production ceases, and the mycelium begins the development of 
sexual structures. The detailed study of the development of these struc- 
tures cannot well be followed out in this laboratory exercise. (See de 
Bary, Morphology and Biology of the Fungi, p. 226, fig. 107.) _ ; 
The structure of the perithectum may, however, be readily studied. 
Examine the specimens provided under the binocular microscope. oB- 
SERVE :-—_ 
