212 REPRODUCTION OF THE BLACK MOULDS 



their relationship to any group of the algae is not known. Some 

 of the hyphae of the mycelium creep over the food supply and 

 send into it short branches which serve as organs of absorption 

 while other rather thicker hyphae grow away from the mycelium 

 and reach up into the air (Fig. 134). The tips of these erect 

 hyphae enlarge owing to the accumulation of protoplasm and 

 numerous nuclei in them, and finally become spherical, forming the 

 sporangia. The contents of the young sporangia differentiate into 

 a denser peripheral portion containing the bulk of the nuclei and 

 a more watery central region. A layer of large, round vacuoles 



Fig. 134. Habit of growth of the black mould, Rhizopus: s, sporangia; 

 r, absorbing branches of the mycelium. 



now appears between these two regions and, owing to the flatten- 

 ing out and fusion of these vacuoles, a cleft is formed that ulti- 

 mately separates the peripheral from the central protoplasm. 

 Next furrows advancing inward from the sporangial wall and 

 outward from the cleft divide the peripheral protoplasm into 

 numerous small portions, each part containing several nuclei. 

 These bodies round off, secrete a rather thick smoky black wall 

 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 forming a dome-like structure 

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

 walls of the sporangia readily dissolve in the presence of moisture, 

 owing to their mucilaginous character, and thus allow the spores 



