64 



MORPHOLOGY 



colorless Vaiicheria. The representative genus Saprolegnia contains 

 saprophytic species found on dead bodies of crustaceans, water insects, 

 etc., and also parasitic species attacking fishes, frogs, etc. One species 

 that attacks the eggs and young of fishes is very destructive in hatcheries. 

 The asexual reproduction resembles that of Cladophora, sporangia 

 developing as terminal cells and producing vast numbers of biciliate 



zoospores, which escape through a termi- 

 nal pore (figs. 155, 156). The oogonium 

 is a spherical cell, borne terminally or 

 laterally, and contains one to several 

 eggs. The antheridium is tubular in 

 form, arising from another hypha or 

 from the same one, and grows up in 

 contact with the oogonium in various 

 ways, sometimes curving about it. A 

 small fertilizing tube sent out by the 

 antheridium pierces the wall of the 

 oogonium, reaches an egg, and through 

 it the contents of the antheridium are 

 discharged (fig. 157). Fertilization re- 

 sults in heavy-walled oospores, which 

 upon germination form new hyphae 

 directly. 



Figs, i 5 5-157. — Saprolegnia: 

 155, terminal cell producing zoo- 

 spores; 156, free zoospores; 157, fer- 

 tilization, showing oogonium con 



Parthenogenesis . — ■ Saprolegnia is notable for 

 the frequent and perhaps usual occurrence of 

 parthenogenesis. All stages of abortion of the 

 male apparatus have been observed : as, for 

 taining several eggs, and antheridial example, the antheridial tube reaching the 

 tube piercing the oogonial wall. egg but remaining closed ; the tube piercing 



the wall of the oogonium but not reaching the 

 egg; the suppression of an antheridial tube; or even the suppression of the 

 antheridium. In all these cases the eggs develop as if fertilized, and produce new 

 plants. 



Experiments. — The culture of Saprolegnia under experimental control has 

 succeeded in determining the conditions that favor vegetative activity, zoospore 

 formation, and gamete formation. If well nourished, the plant vegetates in- 

 definitely; if it is starved, as by removal to pure water, zoospore formation is in- 

 duced; if the temperature is lowered, or if the plant is transferred to a solid substra- 

 tum, conditions forbidding swarm spores, oogonia are formed. 



Monoblepharis. — This is a form similar to Saprolegnia in many respects, being 

 an aquatic saprophyte on decaying plants ; but it is chiefly interesting as the only 



