SECT. I 



CRYPTOGAMS 



383 



forming swarm-spores. A similar transformation of sporangia into conidia is also 

 found in other Peronosporeae as a result of their transition from an aquatic to a 

 terrestrial mode of life. 



Plasmopara viticola, an extremely destructive parasite, also produces copiously 

 branched sporangiophores and occasions the " False Mildew " of the leaves and fruit 

 of the Grape-vine. Albugo Candida { = Cysto2)us candidus), another very common 

 species, occurs on Cruciferae, in par- 

 ticular on Capsella bursa pastoris, 

 causing white swellings on the stems. 

 In this species the sporangia are 

 formed in long chains on the branches 

 of the mycelium under the epidermis 

 of the host plant, and produce in 

 water numerous swarm-spores. 



The sexual organs of the Perono- 

 sporeae resemble those of the genus 

 J'auchcria (Fig. 288). They arise 

 within the host plant — the oogonia 

 as spherical swellings of the ends of 

 certain hyphae, the antheridia as 

 tube -like outgrowths arising as a 

 rule just below the oogonia. Both 

 are cut otf by transverse walls and are 



B 



multinucleate (Fig. 311). The several 

 species exhibit interesting differences 

 as regards the nuclear changes. In 

 Peronospora parasitica, . A Ibugo Can- 

 dida, and A, Lepigoni, Pythium, 

 Plasmoptara, and Sclerospura, a single 

 large central egg-cell or oosphere be- 

 comes differentiated in the protoplasm 

 of the oogonium ; this contains a 

 single nucleus in a central position, 

 while the remaining nuclei pass into 

 the peripheral layer of protoplasm 

 (periplasm). The antheridium noAv 

 sends a process into the oogonium, 

 which at its apex opens into the 

 oosphere and allows a single male 

 nucleus to pass into the latter (Fig. 

 311, 2). The oosphere then becomes 

 surrounded with a cell wall (Fig. 

 311, 3), and nuclear fusion takes 



place, while the periplasm is utilised in forming the outer membrane of the spore 

 (episporium). In Peronospora parasitica the ripe oospore has a single nucleus, in 

 Albugo it becomes multinucleate as a result of nuclear division. In Albugo Bliti 

 and A. portulacae there is also a central oosphere surrounded by periplasm, but the 

 oosphere contains numerous nuclei, which fuse in pairs with a number of male 

 nuclei entering from the antheridium. A multinTicleate oospore thus arises from 

 the compound egg-cell. The behaviour of these two species can be regarded as 

 primitive, the uninucleate oosplieres of the first-named forms having been derived 



Pio. 310. — A, Surface view of thf> epirlermis of a 

 potato leaf, ■with sporangiophores of I'hytophthura 

 infestans projecting from the stoniata (x !I0) ; B, 

 a ripe sporangium ; C, another in process of 

 division; D, a swarm-spore. (B-D x 540.) 



