78 THE SAPROLEGNIACE.E OF THE UNITED STATES, 



respect. This points strongly to some attractive and variable constituent of the sur- 

 rounding medium as the cause of the normal exit of the spores.* The aimless wan- 

 dering and frequent failure to escape of the last few spores in a Saprolegnia sporan- 

 gium may be attributed to the fact that the water from outside has so far filled the 

 sporangium that the difference between the media within and without the sporangium 

 has become practically neutrahzed by their mingling. That the attractive- force is 

 normally very strong is shown by the following observation. A sjiorangium of 

 AcTilya Americana had developed abnormall}' three escape papillae, one near the apex, 

 one near the basal wall, and one midway between the others, all on the same side. 

 The middle month was the first to open, and the spores rushed out in normal fashion 

 until about a third of them had escaped. Then the other two mouths opened almost 

 simultaneously, and the spores nearest them broke away from the column of which 

 they formed a part, and crowded out through the new openings. Thus the force 

 causing their exit was stronger than their tendency to cling together, and drew those 

 within its range away from the main body. Whether the attraction is due wholly to 

 the presence of free oxygen seems still open to doubt, although Hartog's experi- 

 ments ('88) show it to be a factor of prime importance. It is true that failures of the 

 sporangia to empty normally occur often in old cultures in which the oxygen may 

 well be largely exhausted, and very often in cultures which have become overrun by 

 Bacteria, Infusoria, and other foreign organisms. But cases occur which do not seem 

 explicable on this basis alone. For example, in cultures on the slide with a compara- 

 tively small quantity of water, though quite sufficient for the swarming of zoospores, 

 sporangia often fail to become emptied, yet here there can be no suspicion of any 

 lack of oxygen, but rather an unusual abundance of it. 



In spite of the great differences in the size of the sporangia in different species of 

 Sajirolegniacece,, and even in the same species, the size of the zoospores, as measured 

 after their encystment, varies but little, except in 8. anisospora, which is not yet 

 known to be American. The encysted spores are quite exactly spherical in all the 

 American species studied, and are almost always between 8 and 12,a in diameter. 

 Those of a given species may show a tendency toward one or the other of these 

 extremes, but they present no constant characters in this respect, and are of abso- 

 lutely no diagnostic value. Each spore contains a single nucleus, one of the original 

 nuclei of the sporangium, of a nearly globular form, but otherwise like those of the 



* On the other liancl, llie mouths of some Saprolegnia sporangia often resemble very exactly the opening formed 

 in a glass tube with one closed end, by forcing air into the open end while a small area on the wall is softened in a 

 flame. The likeness is so striking as to suggest at once the action of an impulsive force from within the sporangium 

 (see Fig. 50). 



