290 PART III. THE CLASSIFICATION OF PLANTS. 



celia. The mycelium becomes completely exhausted in forming the gonidia, so 

 that when this process is completed, both the fungus and the insect are dry and 

 shrivelled. 



In some cases few or no gonidia are formed, but zygospores or azygospores. 

 The zygospores are produced by the protusion of transverse protuberances be- 

 tween two segments of adjacent hyphae ; the walls of these protuberances are 

 absorbed at the point of contact, the contents fuse, and the product surrounds 

 itself with a proper wall, and constitutes a zygospore (Entomophthora ovispora, 

 and curvispora). In other species spores quite similar to the zygospores, termed 

 azygospores, are formed, but without conjugation ; the azygospores are formed 

 parthenogenetically. On germination (as observed in Entomophthora Grylli), 

 the zygospore gives rise to a hypha, the promycelium, which forms a gonidium 

 at its apex. 



The life-history of the sexual forms shows a clear alternation of generations. 

 The plant is the gametophyte which, as is so commonly the case among the 

 Thallophytes, reproduces itself asexually by means of gonidia, and gives rise to 

 zygospores as well. The promycelium is the sporophyte, as it never can de- 

 velope sexual organs and produce zygospores. By analogy, in those forms 

 which are not known to produce zygospores, the plant is a gametophyte, and 

 the sporophyte is absent. 



Section B. Oomycetes. 



This section of the Phycomycetes includes the following orders : 



Order 1. Ancylistacese : body generally unbranched ; oogonia 

 intercalary; contents of oogonium not differenti- 

 ated into oosphere and periplasm; pollinodium 

 functional. 



Order 2. Peronosporaceae : body branched ; oogonia terminal 

 or intercalary ; contents of oogonium differentiated 

 into oosphere and periplasm ; pollinodium func- 

 tional. 



Order 3. Saprolegniaceas : body branched ; oogonia generally 

 terminal, rarely intercalary ; contents of oogonium 

 not differentiated into oosphere and periplasm ; 

 pollinodium absent, or, if present, functionless. 



Order 1. Ancylistaceae. This order consists of a few forms which are 

 parasitic on freshwater Alga3. The body is simply a tube, lying in the cell of 

 the host. It becomes branched only in some forms (Lagenidium), and then 

 only in connexion with the development of reproductive organs. It eventually 

 becomes septate transversely, and each segment becomes a reproductive organ, 

 either sexual or asexual, so that these organisms are holocarpic and mono- 

 carpic. 



The asexual organs are gonidangia, which, in most cases, give rise to zoo- 

 gonidia ; but in Ancylistes this is not the case, where the gonidangium germin- 

 ates as if it were a gonidium, sending out a bypha which makes its way into a 

 host. 



