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 th6 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- 



