PHYCOMYCETES 



143 



portions. The hyphae are apparently intercellular at first and after- 

 wards intracellular. 



Terminal or intercalary spherical sporangia are sparingly pro- 

 duced. These are usually persistent, and may be from three to 

 five times the diameter of the hyphse. During germination a 

 short tube is developed at one side, and through this the my- 

 celium migrates, forming a kind of swarm sphere within which it 

 breaks up into bean-shaped masses, which are set free as zoospores 

 with two lateral (Hesse figures only one) cilia. Thicker walled 

 sporangia-like bodies, called conidia, are also produced. These 

 are deciduous, and germinate immediately by forming zoospores. 



Fig. 44. Mycelium of Pythium invading Tissues 



Thick-walled resting conidia also appear, and these eventually 

 germinate by means of a germ tube. 



The oogonia, or egg-bearing gametes, are formed either as 

 terminal or intercalary enlargements, and are of much the same 

 form as the sporangia. When two or three times the size of 

 a hypha they are cut off from the vegetative cells. The proto- 

 plasm is gradually differentiated into a central denser portion, 

 ooplasm, or oosphere, and a less dense peripheral "periplasm." 

 A coincident development of an antheridium or male gamete 

 takes place, the latter arising either as the enlarged terminal 

 portion of a separate antheridial branch (from the same or from 

 a different hypha), or as a lateral cell cut off directly adjacent 

 to the oogonium. More than one antheridium may be present, 

 each coming in contact with the oogonium. From an anther- 

 idium a fertilization tube grows into the oosphere, and through 

 this tube a nucleus and some cytoplasm pass in, and the nucleus 

 fuses with the nucleus of the oosphere (Fig. 45). This process 



