xm] "DAMPING OFF'' OF SEEDLING-TREES. 281 



is carried on to a cotyledon of one of the seedlings. 

 Let us further assume that this occurs one warm 

 evening in May or June. During the night, as the 

 air cools, the cotyledon will be covered with a film or 

 drops of water, and the conidium will germinate and 

 allow, say, thirty zoospores to escape. Now, the 

 average size of a conidium is about 1/400 of an inch 

 long by about 1/700 of an inch broad, and we may 

 take the zoospore as about 1/2000 of an inch in 

 diameter ; thus it is easy to see that the film of 

 moisture on the cotyledon is to a zoospore like a 

 pond or a lake to a minnow, and the tiny zoospores, 

 after flitting about in all directions, come to rest at 

 so many distant points on the cotyledon — or some of 

 them may have travelled abroad along the moist stem, 

 or along a contiguous leaf, &c. Before daylight, each 

 of these thirty zoospores may have put forth a 

 filament (Fig. 44» ^J) which bores between the cells 

 of the cotyledon, and begins to grow and branch in 

 the tissues, destroying those cell-contents which it does 

 not directly absorb, and so producing the discoloured 

 disease-patches referred to. Supposing the weather 

 to remain damp and warm, some of the hyphse may 

 begin to emerge again from the diseased and dying 

 seedling on the fourth day after infection— or at any 

 rate within the week— and tlii§ may go on hour after 



