OEDEE SAPEOLEGNIALES 107 



short branches or may arise as enlargements of the oogonial stalk, just 

 below the oogone. When arising at a distance these antheridial hyphae 

 are attracted to the oogones apparently by a secretion from the latter. 

 J. R. Raper (1939, 1940) shows evidence that these are in the nature of 

 hormones. In heterothallic species of Achlya he demonstrated that a 

 secretion from the male plant stimulates the formation of oogones on the 

 female plant and that in turn the substances given off by the developing 

 oogones lead to the production of antheridial branches in the male plants 

 and, probably, guide chemotropically the direction of their growth. 



Upon reaching the oogone the usually somewhat enlarged tip of the 

 antheridial branch flattens against it and, if it has not already occurred, 

 a septum is produced, separating the antherid from the supporting 

 hypha. The antherid is usually plurinucleate and the nuclei may divide 

 again. Eventually most of them degenerate. At or near the center of the 

 contact surface a tube grows from the antherid through the oogone wall 

 to the egg, or if there are several eggs this conjugation tube may become 

 branched so that one antherid may fertilize several of them. Couch (1924) 

 showed that in Leptolegnia caudata de Bary an opening is dissolved 

 between the antherid and oogone permitting fertilization without the 

 formation of a conjugation tube. According to Kevorkian (1925) this is 

 true also of Apodachlya hrachynema (Hild.) Pringsh. while Cooper (1929) 

 also demonstrated this for Brevilegnia diclina Harvey. After the entry 

 of a single sperm nucleus into the egg the latter secretes a definite wall 

 which may become thick, with a smooth or rough exterior. The union of 

 nuclei does not occur until much time has elapsed. Although it has not 

 been demonstrated it is assumed that meiosis occurs at the germination 

 of the oospore. This usually occurs by the formation of a hypha which 

 may develop to form a new plant or which may produce a zoosporangium. 

 In many species of this order the oospores develop parthenogenetically. 

 The formation or nonformation of antherids depends upon the conditions 

 of nutrition, temperature, etc. It has been asserted that in some cases 

 antherids, though present, may not function. 



It is difficult to make a decision as to whether the pluriovulate con- 

 dition should be considered the more primitive or a derived condition 

 in this order. If the ancestral forms were fungi whose female gametangia 

 contained several large motile eggs (as in Allomyces), we would expect 

 the more primitive Saprolegniales to be pluriovulate, but if we look to 

 the Lagenidiales for the stock whose evolution led to the Saprolegniales, 

 we find that they have but a single egg. If we look to the Siphonales, we 

 find that in these green algae in the genus Vaucheria the oogone contains 

 but a single egg while there are other members of that group in which the 

 oogone contains several eggs. The author is inclined to favor the hypothe- 

 sis that the pluriovulate condition is derived from the uniovulate con- 



