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A TEXTBOOK OF THEORETICAL BOTANY 



types exist, which are called " plus " and " minus " respectively. Zygospores 

 can only be formed when mycelia of both types meet. There is no morpho- 

 logical distinction between the mycelia, though obviously there must be 

 a physiological one. Such a condition, where the thalli are morphologically 

 alike but physiologically different, is termed heterothallism. It is widespread 

 in the Fungi, and many species are found to exhibit this phenomenon. 



When the hyphae of the two strains come into contact, short lateral 

 branches, called progametangia, are developed, which adhere together 

 by their tips (Fig. 236). They swell and a transverse septum appears in 



Fig. 236. — Mucor rtiucedo. Sexual reproduction. A, Progametangia 

 adhering. B, C and D, Diflferentiation of gametangia and sus- 

 pensors. E, Formation of fusion cell. F and G, Development of 

 zygospore. H, Mature zygospore. 



each, cutting off a terminal cell or gametangium. The remainder of the 

 progametangium is known as the suspensor. These gametangia are 

 multinucleate, and when mature the wall between them breaks down and 

 a single fusion cell is formed. The wall of the fusion cell becomes thickened 

 and dark coloured, and develops a rough warty surface. This cell is now 

 termed the zygospore (Fig. 237). The thick wall forms the exospore, 

 while within it is a thin membrane called the endospore. The suspensors 

 wither and the zygospore is set free (Fig. 238). 



