CHAPTER III. SPORES OF FUNGI. 83 



has in many cases been formed some time before and gelatinously thickened, 

 does not swell in the water before this moment must be due to its not having yet 

 undergone the change which takes place in it after the spores are matured, supposing 

 both phenomena to have a common cause or the cause of the change to lie in the 

 ripe spores. In the latter which is the more probable case we are almost compelled to 

 suppose that some secretion must proceed from the spores, which acts as a ferment 

 altering and dissolving definite portions of the wall which had been previously 

 prepared. The same view will apply with slight modification to the substance inside 

 the persistent firm wall of the sporangium which is capable of swelling and takes an 

 active part in the expulsion of the spores, and to the discharge of the zoospores of 

 many of the Algae. 



On the special features in the formation of the zoospores of Pythium, the 

 formation of small heads in Achlya, Aphanomyces and Achlyogeton, and the coating 

 of the spores in these genera and in Dictyuchus, which cannot be further described 

 in this place, see section XL and the special literature there cited. 



b. The upper and largest portion of the outer wall of the spherical sporangia of 

 Mucor (including Thamnidium, Rhizopus, Absidia, Phycomyces, &c) and Mortierella 

 is changed when the spores are ripe into a substance which dissolves in water, and in 

 most of the mucor-forms is incrusted with a thin spiky coating of calcium oxalate. 

 The presence of the smallest quantity of water causes the wall and the substance 

 between the spores which is present in greater or less abundance to dissolve and 

 liberate the spores (see p. 75). The lower portion of the outer wall which surrounds 

 the point of insertion does not participate in these changes and remains after the 

 dissolution of the rest of the sporangium as a ring or collar round its insertion ; the 

 basal wall is also persistent and forms in Mucor the strongly convex or even 

 vesicular structure known as the columella. 



In the allied genus Pilobolus the sporangium has at first the shape and structure 

 and even the oxalate incrustation of that of Mucor. The upper and larger portion of 

 its outer wall is very firm and of a bluish black colour ; a comparatively narrow 

 annular zone round the point of insertion is more delicate and colourless. The mass 

 of spores contained in the sporangium is at first surrounded, especially at the point 

 of insertion of the sporangium, by a colourless gelatinous layer lying between the 

 spores and the wall, and endowed with great capacity for swelling in water. This layer 

 appears to be developed to a greater or less extent according to species, but whether 

 it is originally a part of the wall of the sporangium or formed like the spores from 

 the contents of the sporangium is at present uncertain. If water reaches the thin 

 basal zone of the outer wall it penetrates through it and causes the gelatinous layer 

 which lines it to swell up at once, and the wall of the sporangium is consequently 

 ruptured round the point of insertion and carried upwards by the continuously 

 swelling substance. It is not known whether the membrane is still intact when 

 the water makes its way through it, or whether fissures for the admission of the water 

 are previously formed in it as the result of changes of form and varying moisture 

 after maturity. In species like Pilobolus anomalus, Ces. (Pilaira, v. Tiegh.) with 

 very long filiform sporangiophores, nothing further happens beyond the gradual 

 solution of the gelatinous layer and the breaking up of the mass of spores ; but in 

 most species (P. crystallinus, P. oedipus, &c.) the ripe sporangium is abjected from 



G 2 



