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 
Tipe 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. 
6. 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 swellingin 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 sporangiüm 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 
