OOMYCETES. 



107 



conidia. Besides the Potato-fungus (see above), Ph.fagl belongs to this group ; 

 it developes oospores very abundantly, and does great harm to seedlings of the 

 Beech, Sycamore, and Pine trees. Peronospora generally has conidiophores 

 which are repeatedly forked, and bear a conidium on each of the most extreme 

 ramifications. Many do great harm to their host-plants. P. viticola, on Vines, 

 and P. nivea, on umbelliferous plants, have swarmspores, which are absent in 

 the following species of this genus : P. sparser, on Roses ; P. gangliformis, on 

 composites ; P. alsinearum, on Stitchwort ; P. parasitica, on cruciferous plants ; 

 P. viciee, on Vetches and Peas ; P. schachtii, on Beets ; P. violacea, on the 



FIG. 93. A fly overgrown 

 with Sctprolegnia, 



FIG. 94. Formation of swarmspores in a Saprolegnia : 

 a germinating swarmspores. 



flowers of Scaliosa ; P. radii, on the ray-florets of Matricaria. Cystopus (Albugo) 

 has the conidia developed in chains, which form a cohesive white layer under- 

 neath the epidermis of the host-plant. Cy.itnpus Candidas, on cruciferous plants, 

 especially Shepherd's Purse and Brassica ; the germination commences on the 

 cotyledons, and from this point the mycelium developes together with the host- 

 plant ; C. cubicus, on the leaves of Composite. 



Order 3. Saprolegniaceae, Water-Fungi which live as sapro- 

 phytes on organic remains lying in water, for instance, on dead 

 flies (Fig. 93), worms, remains of plants ; but they may also make 

 their appearance on living animals, being 

 frequently found, for example, on the 

 young trout in rearing establishments. 



The thalhis is a single, long and branched 

 cell. It has one portion which serves as 

 root, and lives in the substratum, where it 

 ramifies abundantly for the purpose of ab- 

 sorbing nourishment ; and another portion 

 projecting freely in the water, and sending 

 out hyphee on all sides (Fig. 93). The 

 asexual reproduction takes place by swarm- 

 spores (Fig. 94), which are developed in 

 large sporangia; these swarmspores gener- 

 ally possess two cilia, and on germination FlG - 6--Oogonium with 



two antheridia, Achlya race- 

 grow into new plants. The entire proto- -mesa. 



