ALTERNATION OF GENERATIONS. 85 



nuclear fusion in the ascus had at first no sexual significance, but arose 

 in connection with a process of inhibited cell and nuclear division inci- 

 dent to the relatively excessive nutrition supplied to the ascus, the rela- 

 tions of the young ascus, teleutospore, and basidium become at once 

 intelligible. They are all to be interpreted morphologically as spore 

 mother cells. In the ascus the triple division of the nucleus is necessi- 

 tated by the fact of the two nuclear fusions from which it has arisen. 

 But in the teleutospore and basidium, since the process of conjugate 

 division has worked back, as described above, from its origin at the 

 close of the sporophyte generation to the fertilization with which it 

 began, and one nuclear fusion has thus disappeared, we find the normal 

 double division which occurs elsewhere in spore mother cells. The 

 disappearance of a nuclear fusion in the egg would thus be correlated 

 with the disappearance of a third division in the spore mother cell. 

 The conditions in the rusts and the conditions in the mildews afld 

 Pyronema thus represent two stages in the evolution of a sporophyte in 

 the higher fungi, of which that shown in the ascocarp is the more primi- 

 tive. There is, however, as I have pointed out above, no sufficient 

 evidence in this to lead us to infer that Ascomycetes and Basidiomycetes 

 must be forced into the same phylogenetic series. 



If it is true that the rusts have arisen from the red algae and the 

 fusion in the teleutospore is to be interpreted as I have suggested, we 

 must probably assume, as noted above, that in some of their ancestral 

 forms two nuclear fusions are present in the life cycle. In their present 

 condition it is plain that they are much changed from the red algae. 

 We have nothing in the red algae which can be directly compared to 

 such series of carpogonia, each with its special trichogyne and forming 

 a single row of spores, as Blackman assumes to represent a stage 

 just preceding the present condition of the secidium. Blackman has 

 certainly strengthened the evidence that the spermatia are sexual cells, 

 and that they once functioned in fertilizing a trichogyne-like organ. 

 This conclusion, however, leads us to assume some still more primitive 

 condition than the sorus of carpogonia which he conceives. We must 

 believe that at some period the aecidium was more like a cystocarp and 

 that a single trichogyne sufficed for the fertilization of the entire struc- 

 ture, whether by ooblastema filaments, as in many of the red algae 

 to-day, or by the development of a common procarpic hypha from 

 which, when fertilized, the entire spore mass of the aecidium arose in a 

 fashion analogous to the method of origin of an apothecium of Collema. 

 The discovery by Richards (80) of what is apparently a carpogonial 

 branch in the young aecidium cup of Uromyces caladii and other forms 



