262 



GENERAL BOTANY 



or sporangium bearer. A cell wall next appears, separating the 

 young sporangium from the hypha which bears it, thus forming 

 a greatly swollen terminal and spherical sporangial cell which 

 is filled with dense cytoplasm and many nuclei. This sporangial 

 cell now expands with great rapidity, and with its expansion the 

 wall separating it from its hyphal stalk grows in surface area and 

 assumes a convex form, protruding into the growing sporangium 

 until it comes to occupy fully one half or two thirds of the spo- 

 rangial cavity (5), Avhen it is called the columella. The mold, if 



Youny sporangia Mature 



I M^l sporangium 



FIG. 141. Development of the sporangia and spores in Rhizopus 



a, young sporangia occurring as terminal enlargements of aerial hyplue; 6, older 

 sporangium with columella and spores differentiated ; c, spores being shed by rup- 

 ture of sporangium wall ; d, early stage of spore germination ; e, later stage, with 

 branched hyphal filament 



observed at this stage with a hand lens, appears to be dotted 

 with round white or gray balls, which are the young sporangia. 

 Meanwhile internal cell processes which are to result in the for- 

 mation of the spores have been going on in the cytoplasm of the 

 sporangium. As the spores mature they secrete a black pigment, 

 so that the sporangia become darker with age. Finally the spo- 

 rangium cavity is filled with innumerable dark spores, and the 

 plant now presents the familiar appearance of the black molds, 

 owing to the large number of black sporangial heads which cover 

 its surface. When the sporangia are ripe, the sporangium wall 

 ruptures by drying, and the light spores are widely scattered by 

 wind in the open and by air currents in dwellings. 



