614 AQUATIC PHYCOMYCETES 



increased knowledge of this genus to a point far beyond that attained 

 for any other member of the order. For example, it is now safe to say 

 that far more is understood concerning the geographic occurrence of 

 Allomyces than of any other aquatic phycomycete. By the use of water 

 cultures prepared with soil samples collected from various localities 

 throughout the world and baited with split hempseed, the known dis- 

 tribution of the species has been enormously increased (see especially 

 the work of Emerson [1941 ],F. T. Wolf [1941a], and Gaertner [1954b]). 



MORPHOLOGY AND DEVELOPMENT 



The sequence of development of the thallus in Allomyces, whether 

 asexual or sexual, follows a single pattern. The swarmer, be it zoospore, 

 zygote, or flagellated planont from the resting spore, comes to rest after a 

 period of motility, loses its flagellum, and encysts. A germ tube is then 

 produced at one pole, which branches to lay down the first elements of 

 the rhizoidal system. Soon after, a second, much broader, germ tube 

 grows from the spore body at the pole opposite the point of origin of 

 the rhizoids (Fig. 41 G, p. 622). This second tube ultimately forms the 

 basal cell, from which arises distally in regular dichotomous fashion a 

 fan-shaped extraordinarily symmetrical complex of hyphae. At occa- 

 sional intervals along the branches of the young thallus and near the 

 primary diverticulum of the basal cell there are formed discontinuous 

 rings of refractive material which project inwardly from the hyphal wall. 

 These, by centripetal accretion, elongate to produce wheel-like pseudo- 

 septa (Coker, 1930). Additional material may be laid down to form a 

 plate punctuated by large radiating triangular openings. Constrictions 

 of the hyphae frequently occur wherever the pseudosepta are located 

 and give a jointed appearance to the thallus. The primary reproductive 

 organs, whether zoosporangia, resting spores, or gametangia, are form- 

 ed at the tips of hyphal branches, from which they are separated by 

 true cross walls. Secondarily formed zoosporangia and resting spores 

 terminate hyphal branches which, in sympodial fashion, have grown up 

 from beneath the primary organ. Zoosporangia and, rarely, resting 

 spores may also develop in basipetal succession along a hypha. 



On the sexual plant the primary gametangia ordinarily appear in 

 pairs, a male and a female, at the tips of the hyphae (Fig. 40 A-C, p. 616), 



