2o8 A TEXTBOOK OF THEORETICAL BOTANY 



Saprolegnia 



The separation of the species and even the genera of the family Sapro- 

 legniaceae is by no means easy. The genera are separated mainly by the 

 method of emergence of the asexual zoospores, the species by minor details 

 of size in the reproductive organs. 



In nature these Fungi grow chiefly on decaying plant remains lying at 

 the bottom of ponds and slow-flowing streams, or on humus in soils. Recent 

 work seems to indicate that while some species of Saprolegnia occur regularly 

 in water, others are more typically soil-inhabiting organisms living on humus. 

 Many of these species occur in water only as a result of their reproductive 

 bodies being washed out of the soil by the rain ; though if circumstances 

 are favourable they will continue to develop under aquatic conditions. The 

 name " Water Moulds," which is collectively given to this group, is therefore 

 somewhat of a misnomer. They are generally obtained in the laboratory by 

 placing dead flies or the cut seeds of hemp or cress in pond water, when, 

 after a few days, a copious growth of the Fungus will appear on the bait. 

 The Fungus produces a transparent whitish film over the substratum, 

 which consists of a large number of very fine hyphae, collectively spoken 

 of as the mycelium. These hyphae are very long and are not divided 

 by septa into separate cells, hence they must be regarded as coenocytic. 

 Only the sex organs are cut off by septa. The hyphae may be branched or 

 simple, and the degree of branching varies very much among different species, 

 and according to the conditions under which the Fungus is growing. In 

 fact, not only the appearance but the whole sequence of events which make 

 up the life history of Saprolegnia can be considerably altered according to 

 the environmental conditions. 



Saprolegnia grows rapidly, and under favourable circumstances asexual 

 reproduction occurs within a few days of the appearance of the Fungus on 

 the substratum. After a few days of active asexual reproduction the sex 

 organs appear, and this is followed by a falling off in the formation of asexual 

 bodies. 



Asexual Reproduction 



The asexual reproductive organs are zoosporangia, which are produced 

 terminally at the ends of the hyphae (Fig. 200). They are long tubular 

 structures, tapering at the tip. The contents become very dense as protoplasm 

 migrates from the hypha towards the tip. Finally the structure is cut off 

 at its base by a septum. Then the contents divide up into a large number 

 of uninucleated zoospores (Fig. 201), each of which is pear-shaped and 

 provided with a pair of apical flagella. When mature they escape one after 

 another through an opening in the top of the sporangium. They swim away, 

 but their activity is of brief duration, for after a short while they withdraw 

 their flagella and become spherical. After a period of rest they again become 

 active, only this time the cells are kidney-shaped and are provided with 

 two lateral flagella. This phenomenon, consisting of two motile stages 



