344 



BOTANY 



Sub-Class 1. Oomycetes 



To the Oomycetes belong a large number of genera which live either 

 in water upon decaying organisms, or on land, parasitic on higher 

 plants. In the formation of their sexual organs, oogonia, and an- 

 theridia, as well as of asexual swarm -spores, they often show a 

 striking resemblance to certain of the green filamentous Algae. 

 Within this group, however, a reduction of all sexual differentiation, 

 even to its complete disappearance, may be observed. 



1. The Monoblepharidineae may be placed first in a series illustrating gradual 

 reduction, — the only family of all the Ryphomycetes which still produce well- 

 developed spermatozoids in antheridia. They comprise but two genera, with 

 altogether only three species, and have a unicellular, branched mycelium, which 



Fig. 265. — Monoblepharis sphaerica. End of filament with terminal oogonium (o) and an antheridium 

 (ft) : 1, before the formation of the egg-cells and spermatozoids ; 2, spermatozoids (s) escaping 

 and approaching the opening of the oogonium ; 3, osp, ripe oospore, and an empty antheri- 

 dium. (After Cornu, x 800.) 



lives in water upon decaying organic matter. Asexual reproduction is effected by 

 means of uniciliate swarm-spores, formed in large numbers in terminal sporangia. 

 The sexual organs have the form of terminal oogonia and antheridia borne at the 

 tips of certain hyphiE ; the former contain one egg-cell, and the latter numerous 

 uniciliate spermatozoids (Fig. 265). The spermatozoids make their escape through 

 an opening in the antheridium and fertilise the egg-cell, which then becomes trans- 

 formed into a spinous oospore. In the formation of their sexual organs there is an 

 evident resemblance between the Monoblepharidineae and the algal genus Oedogonium. 

 2. In the family of the Peronosporeae a reduction of sexuality is observable 

 in the antheridium, in that its protoplasm, although multinuclear, does not 

 divide into spermatozoids. All the numerous species of the family are parasites. 

 Their profusely branched unicellular mycelium penetrates the tissues of the 

 higher plants, and is frequently the cause of death. In damp climates, 

 certain species occasion epidemic diseases in cultivated plants, and are highly 

 destructive. Thus, the mycelium of Phytophthora infestans, the fungus which 



