WITH NOTES OX OTHER SPECIES. 77 



sure as possible to the surrounding medium, may account for their arranging them- 

 selves in a hollow sphere. 



We may ask here what causes lead to the emptying of the sporangium. The 

 existence of a special expulsive substance which swells strongly on absorbing water 

 was assumed by earlier writers, Strasburger ('80), Biisgen ('82) and DeBary ('84). 

 It was believed that the supposed "cell plates" of the first separation stage swell 

 into an intermediate substance enclosing the spores and expelling them by its rapid 

 and enormous increase in volume when water enters the sporangium. But the exist- 

 ence of this hypothetical substance has been sufficiently disproved along with that of 

 the " cell plates." The only species in which an intermediate substance appears to 

 exist is one in which it could be of no service in expelling the spores, as will be seen 

 later ; namely, ThraustotTieca clavata. Since this species was the one which Biisgen 

 studied in most detail, the reason for some of his views becomes evident. In general, 

 there can be little doubt that the spores of the genera now under discussion leave the 

 sporangia automatically. The preliminary changes in the wall, normally at the 

 apex of the sporangium, which render the escape of the spores possible, involve inter- 

 esting physiological questions which will not be discussed at present. But the 

 nature of the stimulus which causes the spores to avail themselves as quickly as 

 possible of the means of escape provided, may be briefly discussed. It frequently 

 happens in Aclilya and Saprolegnia that the spores do not leiive a sporangium which 

 has opened normally, but become encysted within its interior. This failure to leave 

 the sporangium must evidently be due to the absence of the usual stimulus. Hartog 

 ('87), and before him Cornu i'77), has held that the presence of free oxygen in the 

 water is the determining factor. Rothert ('88) disputes this view, as well as Hartog's 

 statement that such sporangia appear chiefly in poorly aerated cultures. And Hartog 

 has more recently ('88) suggested that the spores may vacate the sporangium to find 

 purer conditions than prevail within it; in other words, to escape from the products 

 of their own metabolism. 



In the first place, it may be remarked that it is much less probable that the 

 hereditary phenomena of spore development within the sporangium should vary than 

 that differences should occur in the external conditions of different cultures, or of the 

 same culture at different times. Since there is no evidence that the formation of the 

 spores presents any abnormality in those sporangia which fail to discharge their con- 

 tents, we cannot suppose that the need of purer conditions exists less in one sporan- 

 gium than in another. But some sporangia with normally developed mouths fail to 

 discharge their spores when others are wholly emptied, though it is more common to 

 find most of the sporangia formed at a given time in the same condition in this 



