456 SYSTEMATIC BOTANY. 



of a mycelium of long, densely interwoven hyphoe, which bears both 

 sexual organs antheridia (male organs) and ooyonia (female organs) 

 and asexual zoosporangia. The contents of the oogonia, when 

 fertilized by the antheridia, are called oospores, and on germinating 

 produce a mycelium which bears first zoosporangia and later the sexual 

 organs. The zoosporangia on bursting produce zoospores which, 

 after a short motile state, come to rest, germinate, and form a 

 mycelium which produces again zoosporangia, and later the sexual 

 organs. The zoospore, therefore, which has an asexual origin is, 

 functionally, of equal value with the sexually produced oospore. 



Oospores 



Zoosporangia Oospores 



Zoosporangia, Oospores. Zoosporangia, Oospores. 



ILLUSTRATIVE GENERA : Pythium, Nees j Saprolegnia, Nees ; Aphano- 

 myces, De Bary ; Achlya, Nees. 



Structure and Life-history. In the forms which Pringsheim calls mon- 

 oecious the antheridia and oogonia are produced beside each other on the 

 same plant ; hut in the others, first antheridia and then oogonia. The 

 oogonia are usually situated at the end of short branches of the mycelial 

 hyphae, and are very rarely interstitial. In the monoecious forms they 

 are globular cells, rich in protoplasm, which is at first equally distributed. 

 In Saprolegnia monoica the cell-membrane is resorbed at numerous places, 

 and has a perforated appearance. At the same time the protoplasm gra- 

 dually separates into several portions, which become rounded off and float 

 together in a watery fluid within the oogonium, each bounded by a 

 smooth superficies which does not consist of cellulose. In Pythium t 

 Aphanomyces, and several species of Saprolegnia the whole of the proto- 

 plasm within the oogonium contracts into one of these globes, which, 

 floating in the watery fluid, takes up its position in the middle of the 

 oogonium. During the formation of the oogonium, the antheridia or 

 antheridium, as the case may be, grow out from the same branch of the 

 mycelium or from neighbouring hyphae in the form of thin, cylindrical 

 crooked twigs, often wound round the stalk of the oogonium. The upper 

 ends adhere to the wall of the oogonium, swell slightly, and become 

 bounded at the base by a septum. At the time of the formation of the 

 globular bodies within the oogonium, each antheridium pushes through 

 the wall one or more tubes, which open at the points and discharge their 

 contents. These contents are minute motile corpuscles, scarcely j^ ff 

 millim. in size, and are the fertilizing spermatozoids. The globular bodies 

 after being fertilized are provided with a cellulose membrane, and receive 

 the name of oospores. 



In the dioecious forms (e. g. Saprolegnia dioica and Achtya dioicd) the 

 oogonia and their globular contents are formed as in the monoecious 

 species. The antheridia, on the other hand, are formed in thick bladder-like 

 protuberances which arise at fixed times on the mycelium, and are divided 



