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A TEXTBOOK OF THEORETICAL BOTANY 



which the large zoospore forces its way. This zoospore is generally inter- 

 preted as a compound structure, resulting from the failure of the protoplast 

 within the sporangium to divide into a number of uninucleate biflagellate 

 zoospores. Zoospores are formed only by aquatic species or by terrestrial 

 ones which have become flooded. They are usually developed during the 



ZOOSPORANGIUM 



I 'J: 



B 



FLAGELLA 



CHLOROPLASTS 



Fig. 77. — Vancheria repens. A to D, Formation of the zoosporangium 

 and liberation of the zoospore. E, Liberated zoospore. F, Structure 

 of the periphery of the zoospore. (After Goetz.) 



night, or they may be induced by a sudden transference from light to darkness, 

 or from running to quiet water. On liberation the zoospores are sluggish 

 and only swim for a short time. On coming to rest the flagella are immedi- 

 ately withdrawn and a thin membrane is secreted around the zoospore. 

 Germination begins with the protrusion of two or more tubular outgrowths, 

 one of which attaches itself to the substratum and forms the holdfast, whilst 

 the other produces the filament. 



Not all species of Vaiicheria produce zoospores. Some develop what are 

 termed aplanospores. These are simple non-flagellate structures which 

 are produced in special sporangia called aplanosporangia. These aplano- 

 sporangia develop as swellings either on the main or lateral branches of the 

 thallus. When mature the aplanospores simply drop out of the aplano- 

 sporangium through a perforation in the wall. This second method of 

 asexual reproduction occurs most commonly among terrestrial species. 



Sexual Reproduction 



In Vaucheria the sexual reproduction is oogamous and occurs most 

 frequently in plants growing on damp soil or in quiet water, but is rarely 

 found in plants growing in streams. The antheridia and oogonia are borne 

 close to one another, either together on the same filament or on a common 

 lateral branch or on adjoining lateral ones. Development of the oogonium 

 begins (Fig. 78) with the formation of a swelling, which becomes filled with 



