RECENT CONTRIBUTIONS TO OUR KNOWLEDGE OF THE AQUATIC PHYCOMYCETES

Summary. From the foregoing account of certain phases of recent researches on the aquatic Phycomycetes, it may be seen that these have been mainly concerned with a determination of what forms are present in nature. There probably exists a large aquatic flora with only a few scattered members of which we are now familiar. Among the Chytridiales, considered the rarest and most perplexing of these fungi, the beginnings of attempts to cultivate them on artificial media have been made. New types of development have been found, many new genera and new species, occurring in both fresh and marine waters, have been described, and a reexamination of established forms has clarified certain moot points in their morphology and life history. The diversity of the sexual processes previously known to occur among the various genera has been reaffirmed, and in the large genus Rhizophidium definite sexuality has been discovered in several species. In the small order of the Blastocladiales, in the genus Allomyces, copulation of anisogamous planogametes has been found to occur. There is exhibited by one species (A. javanicus) a morphological alternation of generations. Two plants are formed, which while they resemble each other in superficial aspect produce different types of reproductive organs, one, male and female gametes, the other, zoospores and resting spores. Several papers have appeared which give details of the morphology and particularly of the process of sexual reproduction of Monoblepharis. The zoospore of this genus has been definitely determined to be posteriorly uniciliate. Although the members of the Blastocladiales and the Monoblepharidales bear little superficial resemblance to each other, the similarity of their zoospores and particularly of their methods of sexual reproduction shows them to be closely related. The Saprolegniales have been found to be of widespread occurrence in the soil. Forms have been found which parasitise other water moulds, capture rotifers, and cause diseases of crop plants. New genera and many new species have been described. Detailed observations on the process of non-sexual reproduction in certain genera have shown that they possess certain optimum temperatures for the discharge of their zoospores. The hitherto unsatisfactorily explained clumping of the emerged zoospores of Achlya and Aphanomyces has been found to be due to the presence of delicate interlocking protoplasmic strands on the spores. Dioecism (heterothallism) has been definitely proved to exist in the genus Dictyuchus and in Achlya bisexualis. It has also been observed in Sapromyces reinschii. Monoecism (homothallism) has been found among certain diclinous species of Saprolegnia, Achlya, and Protoachlya. It has been found that the nature of the developmental process taking place in already delimited sex organs may be considerably altered by changing the environmental conditions. A new order, Leptomitales, has been established to accommodate forms, hitherto placed in the Saprolegniales, in which the thallus is divided by porous cellulin plugs into pseudo-cells. Little of interest has been published concerning the small order Ancylistales, save that a new genus, Lagena, parasitic in the rootlets of certain cereals, has been encountered. Since a very recent monograph of Pythium has brought together the extensive literature of that genus it has seemed superfluous to include it here. Recent work has established that there are a number of species other than those of the Pythium debaryanum-type, which are parasites of flowering plants, that there are many others which parasitise fresh-water algae, and that certain of the latter have been found to be capable, under laboratory conditions, of parasitising the fruits and roots of a few crop plants. A marine species of Pythium has been found. Among the other genera of the order, Pythiomorpha, Pythiogeton, and Zoophagus (a rotifer-capturing form), little work has been accomplished, save the discovery of the sexual stage of the first-named genus.