DEVELOPMENT OF PLANTS 



203 



pressure of the fluid in the hypha, and finally forms a dome-like 

 structure in the sporangium (Fig. 135, DE). The walls of the 



FIG. 135. A, B, early stages in the development of the sporangium of 

 Rhizopus. C, the formation of the cross-wall cutting off the sporangium 

 from the hypha. D, this cross-wall is being pushed up into the enlarging 

 cavity of the sporangium and in E it has become greatly extended, form- 

 ing a sac-like structure, known as the columella. F, the sporangium has 

 ruptured, permitting the scattering of the spores and the columella has 

 formed an umbrella-like structure owing to the loss of its watery contents. 



FIG. 136. Sexual reproduction in the black mould: A, the meeting of 

 the tips of two hyphae. B, later stage, the lower part of the figure shows 

 the cutting off of the tips by transverse walls and in the upper part of the 

 figure the fusion of the contents of the two gametangia thus formed has 

 begun. C, mature gametospore. 



sporangia readily dissolve in the presence of moisture, owing to 

 their mucilaginous character, and thus allow the spores to float 

 off in the air as an invisible dust. The spores will germinate at 



