ORIGIN OF RUSTS AND SMUTS AND HETEROBASIDIAE 647 



Another view as to the origin of the Heterobasidiae (excluding the 

 Uredinales and Ustilaginales) is that they may have arisen from some 

 Ascomycetous form somewhat hke Ascocorticium. In this fungus the asci 

 are developed with the formation of croziers. There are no conspicuous 

 paraphyses and between the wood or bark on which the fructification is 

 produced there are only a few layers of hyphae running parallel to the 

 surface. From the outer hyphae arise the asci. There is no definite limiting 

 margin to this structure which may be looked upon as a much simplified 

 apothecium. It is not known whether the subhymenial hyphae are di- 

 caryotic or monocaryotic or a mixture of the two or whether the mycelium 

 within the substratum is one or the other. Also, it is not known whether 

 spermatia and receptive hyphae are present or how the dicaryophase 

 arises. Therefore we do not have much on which to base a suggestion that 

 a fungus of this structure is a possible forerunner of the Corticium-Yike 

 Basidiomyceteae. If we do make this assumption, we must account for 

 the origin of the basidium (holobasidium in this case) from the ascus. It 

 is known that in all fungi with holobasidia the meiotic division of the 

 diploid nucleus of the young basidium occurs within the latter until four 

 nuclei, or very often eight through a subsequent mitotic division, are 

 produced. Then there arise slender outgrowths from the upper portion of 

 the basidium and these sterigmata enlarge terminally. At this point the 

 nuclei, usually one to each, squeeze through the slender sterigmata and 

 round up in their enlarged ends. Then, and not until then, is another wall 

 laid down within this "spore." This wall is always visible separating the 

 opening of the sterigma from the spore and frequently can be traced all 

 over the inner surface of the outer spore wall, sometimes not tightly 

 appressed to it. In other words, after the nucleus enters the terminal 

 swelling of the sterigma it proceeds to induce the formation of a wall 

 around the nucleus and accompanying cytoplasm. We may then interpret 

 such a basidium as an ascus on which arise evaginations or external 

 pockets within which the ascospores are produced. The so-called basidio- 

 spores therefore may be looked upon as ascospores enclosed within the 

 evaginated ascus pockets. This is somewhat different from the suggestion 

 of Linder by whom the septate promycelium of the teliospore was looked 

 upon as a row of four ascospores, separated by simple septa, not rounded 

 up as occurs in a normal ascus. From each such ascospore a short stalk 

 (sterigma) produces a basidiospore which must then be looked upon as a 

 secondary spore, budded off from the ascospore. Rogers (1932, 1934) looks 

 upon Tulasnella as perhaps derived from an Ascocorticmm-like ancestor. 

 He interprets the four large pockets at the top of the basidium as ascus 

 pockets, each containing the homologue of an ascospore. The spore pro- 

 duced on the slender sterigma from such a pocket is then, as in Linder's 

 hypothesis, not homologous to an ascospore but, as Linder intimated, a 

 secondary spore budded off from the sterigma sent out by the ascospore. 



