190 PHYLUM IV. SIPHONOPHYCEAE 
ing the so-called columella. The protoplasm in the 
enlarged terminal segment (sporangium) divides into a 
large number of minute masses (spores) each of which 
surrounds itself with a cell wall. 
284. The spores are set free in different ways: in some 
cases the wall of the sporangium is entirely absorbed by 
the time the spores are mature; in other cases only por- 
tions of the wall are absorbed, producing fissures of va- 
rious kinds. ‘The spores germinate readily when on or in 
a substance capable of nourishing them, by sending out 
one or two filaments, which soon branch and give rise to 
a mycelium. If kept dry, the spores may retain their 
vitality for months. 
285. Sexual reproduction (generation) may take place 
after the production of asexual spores, but it appears to 
be of rare occurrence in our commonest species. Two 
filaments in the air or within the nutritive medium, in 
contact ‘send out small branches (here regarded as re-_ 
duced sexual organs, the one an antherid, and the other 
an oogone); these elongate and become club-shaped, and 
at the same time become more closely united to each 
other at their larger extremities; a little later a transverse 
partition forms in each at a little distance from their 
place of union; the wall separating the new terminal seg- 
ments is now absorbed, and their protoplasmic contents 
unite into one common mass (the zygote); the last stage 
of the process is the secretion of a thick wall around the 
new mass, thus forming a zygospore, i.e. a resting spore, 
which eventually germinates and sooner or later gives 
rise to a new plant. 
286. In some Black Molds both gametes are formed 
upon different branches of the same mycelium (homo- 
thallic forms, monoecious). In many, however, the 
plants are of two kinds (dioecious), and sexual reproduc- 
