82 



DIVISION I. — GENERAL MORPHOLOGY. 



and exceptional cases the release of the ripe spores is left to chance, there being no 

 special arrangement made for it, and the spores may even germinate inside the mother- 

 cell, the germ-tubes piercing or bursting through its wall, as may be seen in the 

 sporangioles of Thamnidium and its allies. 



The arrangements for the escape of the spores vary in different species. 

 a. The aquatic swarm-spores of the Saprolcgnieac (with one partial exception to 

 be noticed hereafter), of the Peronosporeae and Chytridieae make their exit through 

 a narrow orifice, formed usually at the apex of the wall of the mother-cell by the sudden 

 swelling and disappearance of a circumscribed portion of the wall of -the mature 

 sporangium. The spot is marked out in many species by gelatinous thickening 

 of the membrane before it begins to swell. This is nowhere more conspicuous than 

 in the sporangia of Phytophthora, in some species of Peronospora, and in some of the 

 Chytridieae which have gelatinously thickened terminal papillae ; in other cases, as 

 Saprolegnia, the thickening has not been observed. While the place of exit swells, 

 the entire contents of the sporangium, the mass of spores and the surrounding matter, 

 absorb water and also swell 1 ; and as the lateral walls of the sporangium are but 

 slightly extensible, the spores which lie beneath the place of exit are first squeezed 



out through it and the others follow. 

 The proceeding may vary in individual 

 cases, and it remains for investigation 

 to determine to what extent the spores 

 themselves, the intermediate parting 

 substance (see p. 74) and perhaps also 

 an inner layer of the wall of the spor- 

 angium, participate in the first general 

 swelling caused by the absorption of 

 water. In the cases which have been 

 more carefully examined (Achlya, Sap- 

 rolegnia and Phytophthora, Fig. 42) it can be seen directly that it is the hyaline substance 

 surrounding the spores inside the firm wall which swells the most. It is also observed 

 in most cases that a hyaline layer on the inner surface of the firm wall first comes 

 into prominence, and increases in breadth and pushes the mass of spores towards the 

 middle of the sporangium and the place of exit. The spores, even where as in some 

 cases they show independent movements before they are set at liberty, are now 

 virtually passive, and in Achlya especially they are evidently squeezed together as 

 they escape from the sporangium by the limpid mass which surrounds them. It is 

 therefore in the swelling of this mass that the expelling force resides ; but it is still 

 uncertain whether the mass consists entirely of the original soft partition-layers whi< h 

 must in that case suffer partial dislocation when the spores are discharged, or whether 

 an innermost layer of the wall of the sporangium swells and M>me product from the 

 spons themselves is also added. 



The phenomena connected with this swelling at the plate of exit occur only at 

 a given moment after the formation of the spores is completed, and in water more- 

 over which is perfectly pure and contains free oxygen. That the point of exit, which 



Fig. 42. Phytopthora in/estans, Mont, a sporangium lying in 

 vision is complete, b escape of the 10 (swarm-) 

 spores from the sporangium, c spores in motion, rfthe same come 

 to rest and beginning to germinate. Magn. 390 1 



1 Walz, Bot. Ztg. 1870, p. 689. 



