CLASS PHYCOMYCETES 31 



the whole thallus; they are then only sporangia whose zoospores, possibly 

 because of undernourishment, are no longer capable of further independ- 

 ent development. In the higher forms, however, again like sporangia, 

 they arise as special organs, antheridia, oogonia, etc., which finally engage 

 only a small part of the thallus; in many of these forms, however, condi- 

 tions of nourishment alone determine whether a fundament develops to 

 a sporangium or gametangium. Only in the highest Phy corny cetes is the 

 gametangium so highly specialized that it is unable to change into a 

 sporangium, and degenerates in the absence of a mate. 



These close relationships between sporangia and gametangia increase, 

 so that in the Phycomycetes, the gametangia undergo changes in the 

 course of their phylogenetic development similar to those in sporangia; 

 just as in sporangia, the individualization of spores is absent and the 

 sporangia assume the task of the spores so in the gametangia, individual- 

 ization of the gametes is lacking; hence the gametangia remain coenocytic 

 and allow their contents to fuse without differentiation. Gametangial 

 copulation appears between two coenocytic gametangia instead of copula- 

 tion of gametes. These tendencies appear in the lowest holocarpic 

 families and naturally lead to hologamy ; as the differentiation of gametes 

 is absent, the whole thalli fuse and form a single coenocytic zygote. In 

 the higher forms, gametangial copulation, except in Monoblepharis, is 

 the only type of sexuality known. The original complexes which lead to 

 suppression of individualization of gametes and copulation of coenocytic 

 gametangia, have not yet reached pure gametangial copulation; their 

 reduced activity in the nuclei leads, in the higher forms, to the appearance 

 of privileged sexual nuclei, appearing in both the isogamous and the 

 heterogamous series. 



Corresponding to this variation in development of gametangium and 

 gametes, the zygotes are not entirely equivalent in the Phycomycetes. 

 Externally they appear rather similar, as, almost without exception, 

 biologically they are hypnospores; ontogenetically they may be divided 

 into true zygotes and coenozygotes the product of two coenocytic 

 gametangia. As a conidium corresponds to many zoospores and hence 

 the probability of dispersal is numerically only a fraction of that represen- 

 ted by the totality of these zoospores, a coenozygote corresponds to a 

 large number of true zygotes and has, by analogy, only a fraction of the 

 probability, represented by the totality of true zygotes, to rest over 

 unfavorable times and to reach a favorable substrate. Hence it is 

 significant that in the coenozygotes, the formation of fructifications 

 appears first in the fungi. In the highest Oomycetes, the coenozygotes 

 remain enclosed in the sheath of the female gametangium whose wall 

 undergoes much thickening, and in the higher Zygomycetes they are im- 

 bedded in a sheath of closely intertwined hyphae, rich in reserve 

 materials. 



