CHAPTER XIII 

 OOMYCETALES (CONTINUED) 



Family 4. Chytridiace.e. — This family according to some authors 

 is made to include six families which are here reduced to six subfamilies. 

 It includes fungi of short vegetative duration, which may be a few days 

 in length. The swarm spores quickly give rise to new generations. 

 The resting period is represented in the case of the endophytic para- 

 sites by the time which elapses between the growth of two successive 

 crops of the host plants. The majority of the species of the family 

 are true parasites, partly endobiotic, partly epibiotic, and a few 

 are saprophytes. Half of the plant parasites live in fresh-water 

 algae, nearly as many in flowering plants, some of which are in 

 aquatic plants, some in swamp plants. About ten species are found 

 on marine algse. All species are microscopically small, yet they 

 cause galls, dwarfing, dropsy and crusts of the host plants. The 

 mycelium is absent or in the form of slender protoplasmic filaments, 

 occasionally as distinct one-celled hyphae. The cell, which produces 

 the fruit body, frequently serves as the chief nutritive organ. Later, 

 it divides to form zoospores. The true mycelium has weak develop- 

 ment. The short germ tube merely serves as an organ by which the 

 parasite gains entrance to the host cell, and in the endophytic forms, it 

 disappears quickly, but in the epiphytic species, it serves as an haustor- 

 ium, sometimes with rhizoidal extensions. In the better-developed 

 forms of Cladochytrie^, the slender mycelium serves to carry the 

 fungus from cell to cell of the host. The sporangia are always zoo- 

 sporangia which develop swarm spores, or zoospores. They are thin- 

 walled and quickly mature, or they are thick-walled and form resting 

 sporangia. Sexual spores are formed in only a few types and the differ- 

 ence between antheridia and oogonia is morphologically little pro- 

 nounced. The swarm spores have as a rule a single flagellum, rarely 

 do they have no such locomotory appendages. The sexually produced 

 oospores have the appearance of resting sporangia with the empty 

 antheridium attached as an appendage. Few of these fungi attack our 



116 



