DEVELOPMENT OF PLANTS 



225 



and so become spores (Fig. 135). The numerous sporangia filled 

 with dark spores are the principal cause of the black color of these 

 fungi. 



During the development of the spores a wall is constructed 

 over the central protoplasm, thus often forming a dome-like 

 structure in the sporangium known as the columella (Fig. 135, E). 

 The walls of the sporangia (save at the region in contact with 

 the stalk) 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 germi- 

 nate at once and produce a new plant under suitable conditions 

 of moisture and food, or they will retain their vitality for months 

 if kept dry. As the spores are disseminated, the dome-like struc- 



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

 Rhizopus. C, the formation of the vacuoles which separate the denser peri- 

 pheral protoplasm from the more watery central region. D, the vacuoles 

 are flattening out and the denser protoplasm is becoming separated into 

 multinucleate segment, E, the multinucleate segments are rounding off to 

 form the spores and a wall has formed over the columella. F, the sporan- 

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

 has formed an umbrella-like structure owing to the loss of its watery con- 

 tents. After Swingle. 



ture in the sporangium relaxes, owing to the loss of its watery 

 contents, and assumes an umbrella shape, as seen in Fig. 135, F. 

 You can readily understand why these black moulds are so com- 

 mon by counting the number of sporangia on a small bit of myce- 

 lium and then estimating the number of spores in a sporangium. 

 So numerous and light are the spores that they are carried every- 

 where. It is only necessary to expose a bit of moist bread for a 

 few moments to the air and then enclose it in a damp chamber to 

 secure a luxuriant crop of these plants. An interesting variation 



