346 



BOTANY 



Fig. 267. — Pythium gracite. 



C, after fertilisation ; o, oogonium ; 

 idium. (After De Baby, x 800.) 



from the ends of the hyphse, or sometimes intercalated throughout their length ; 

 the antheridia are developed as septate tubular outgrowths just below the 

 oogonia. The contents of the oogonium become differentiated into one large 

 central oosphere, which is separated by a thin membrane from the peripheral 

 periplasm. In the process of fertilisation the antheridium sends out a tube which 



penetrates the oogonium until it reaches 

 the oosphere. The tube then opens 

 at the apex and the contents of the 

 antheridium pass into the oosphere 

 (Fig. 267). After the fertilisation of 

 the oosphere has been effected, the 

 surrounding periplasm becomes trans- 

 formed into an outer spore-wall or epi- 

 sporium. The oospore then gives rise 

 either directly to a germ-tube (promy- 

 celium) or first develops swarm-spores. 

 In the case of Peronospora parasitica, commonly found on members of the 

 Cruciferae, the behaviour of the nuclei has been more closely investigated. The 

 young oogonium contains numerous nuclei, but although their number is increased 

 (circa 112) by repeated division, only one nucleus is enclosed in the oosphere, the 

 rest remaining in the periplasm. The antheridium is also provided with several 

 (6-12) nuclei, of which only one passes into the egg-cell and fuses with its nucleus. 

 3. The Saprolegnieae, the third family of the Oomycetes, have also a profusely 

 branched unicellular mycelium, but, unlike the Peronosporeac, they live in water, 

 upon the surface of decaying plants, insects, and 

 even upon living fishes. For the purpose of asexual 

 reproduction they develop terminal club-shaped 

 sporangia, which produce numerous biciliate swarm- 

 spores, as in the genus Gladophora (p. 324). In the 

 production of sexual organs, terminal cells of the 

 mycelial hyphee are converted, as in the Perono- 

 sporeae, into spherical oogonia, which give rise to a 

 larger (as many as 50) or smaller number of egg-cells, and 

 less frequently only to one (Fig. 268). The antheridia 

 of the Saprolegnieae are also tubular, and spring 

 from the hyphse, usually just below the oogonia. 

 Applying themselves to the oogonia, they send out 

 fertilising -tubes to the egg-cells (Fig. 268), which fig. 268.— AcMya polyandra. 



An antheridium with two 

 fertilising tubes (b) penetrat- 

 ing the oogonium (o) to the two 

 egg-cells. (After De B<rt, 

 X 350.) 



then become converted into thick -walled oospores. 

 In some Saprolegnieae no antheridia are formed, and in 

 others they only appear occasionally; in such" cases, 

 therefore, all sexual differentiation has been entirely 

 lost. 



i. In the Oomycetes are also included the Ghytridieae, small parasitic Fungi 

 whose unicellular mycelium is only feebly developed, and in some genera is 

 reduced to a simple saccate cell, completely filling the host-cell. Sexual repro- 

 duction has been observed only in a few forms ; they usually multiply by means of 

 asexual swarm-spores formed in sporangia. 



5. The Entomophihoreae, finally, take an intermediate position between the 

 Oomycetes and Zygomycetes. They live, as parasites, in the bodies of insects and 

 caterpillars, etc., and ultimately cause their death. The best-known species is 



