CRYPTOGAMS 



381 



land plants and in some cases on animals. The non-septate mycelium is feebly 

 developed, and is frequently reduced to a single sac-shaped cell inhabiting a cell 

 of the host. Asexual multiplication is effected by means of swarm-spores provided 

 with one or two cilia. In the simplest forms the entire cell becomes converted 

 into the sporangium. Olpidiuni Brassicae (Fig. 319), which is parasitic in the 

 tissues of the base of the stem of young 

 cabbage plants and brings about their 

 death, may be taken as an example. The 

 sporangia in this case have a long neck 

 from which the swarm-spores escape. 

 Thick-walled sporangia which undergo 

 a resting period are also frequently found 

 in the Cliytridiaceae. 



Sexual reproduction is only found 

 in some of the genera, in the form of 

 fusion of an antheridium with an oogou- 

 ium and the production of an oospore. " Fia . 31S.-Acki^ i,ol,,,, nd,-a. Th,- fertilisation of 



two egg-cells, o, of an oogonimn by two tubes 

 from the antheridium a ; ek, nucleus of the egg- 

 cell ; sk, sperm-nucleus; in o* the s.-rtion has 

 not passed through the egg-nuelfiis. (After 

 TROW.) 



The systematic position of the Chytri- 

 diaceae among the Phycomycetes is still 

 doubtful ; the simplest forms appear to 

 have originated in part from Flagellatae 

 and partly from Protococcaceae. 



4. The Peronosporeae ( r>4 ) are parasitic Fungi whose 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. Thus, the mycelium 

 of Phytophtliora, infestans, the fungus which causes the Potato disease, lives in the 

 intercellular spaces of the leaves and tubers of the Potato plant, and by penetrating 



the cells with its short hau- 

 storia it leads to the discolora- 

 tion and death of the foliage 

 and tubers. Sexual reproduc- 

 tive organs have not as yet 

 been observed in this species. 

 Asexual, oval sporangia are 

 formed on long branching 

 sporangiophores which grow 

 out of the stomata, particu- 

 larly from those on the under 

 side of the leaves (Fig. 320), 

 and appear to the naked eye 



" as a white mould. The spor- 



Fio. sw.ulpldium. Hrassim,: A, Thiv,- zoosporangia, th.' angia, at first terminal, are cut 

 contents of one of which has escaped (x 100) B, Zoo- off b transverse walls from 

 s p ores (x o-20). '. Kiting sporangia (X 520). (Aftpr J 



WOROXIX.) the ends of the branches of the 



sporangiophore, by the subse- 

 quent growth of which they become pushed to one side, and so appear to be inserted 

 laterally. Before any division of their contents has taken place, the sporangia (B) 

 fall off and are disseminated by the wind ; in this way the epidemic becomes 

 widespread. The development of swarm-spores in sporangia is effected only in 

 Water, and is consequently possible only in wet weather. In this process the 

 contents of the sporangium divide into several biciliate swarm-spores (C, D). Each 



