IO2 FECUNDATION; HETEROGAMETES. 



and Peronospora parasitica, fusion is retarded, taking place only 

 after the egg has developed a tolerably thick wall about itself. The 

 retarded fusion of the nuclei has already been pointed out for Spiro- 

 gyra, Cosmarium, Closterium, and Basidiobolus, and, as will be 

 seen, it is of frequent occurrence in the plant kingdom. 



ACHLYA AND SAPROLEGNIA. 



The sexuality of the Saprolegniacea is, perhaps, one of the oldest 

 questions in botany still in dispute. The fact that apogamy obtains 

 in so many species has led observers to accept with the greatest reserve 

 any affirmation of sexuality, although based upon observations which, 

 in other groups of plants, would not be questioned as positive proof 

 of a sexual process. 



Pringsheim ('57) was probably the first to attribute to any represen- 

 tative of this group a sexual reproduction, basing his conclusions chiefly 

 upon a study of Saprolegnia monoica. He described the develop- 

 ment of the sexual organs, the penetration of the oogonium by the 

 conjugation-tubes, and their growth inward among the egg-cells. He 

 stated also that the tubes opened and discharged their contents among 

 the eggs. Reasoning from the analogy of Vaucheria, Pringsheim 

 concluded that a real sexual process existed in the species in question. 



Several years later De Bary ('81) combated this view, alleging that, 

 as he did not observe the fusion of the conjugation-tubes with the egg- 

 cells (Saprolegnia ferax and Achlya polyandra), no fecundation 

 took place and that apogamy characterized the entire group. De Bary 

 made a careful study of several species, keeping pure cultures of the 

 same running for several years, and his view, it is safe to say, has been 

 more generally accepted by botanists than that of Pringsheim. 



Pringsheim continued his studies, and in 1882 brought forth addi- 

 tional evidence in support of his view. He described and figured the 

 fusion of the conjugation-tubes with the egg-cells in Achlya poly andr a, 

 and, although his u spermamoeba " were nearly amoeboid parasites and 

 not male gametes, as he persistently maintained, yet his collected 

 observations seemed to furnish as strong evidence in favor of sexuality 

 as that which could be brought against it by his opponents. Since the 

 above mentioned publications of Pringsheim and De Bary the majority 

 of observers dealing with the subject have leaned toward the view of 

 De Bary. 



Within more recent years the subject has been taken up by Hartog 

 ('89, '95) and Trow ('95, '99), with the aid of improved technique, 

 especially on the part of Trow. Hartog reaffirms the doctrine of 



