354 Blackman. — On the Fertilization , Alternation of 
the surface. The absence of this necessity might also lead later to a develop- 
ment deeper down in the tissues in the host, as is found in the typical 
aecidium, where conditions of nutrition would perhaps be more satisfactory. 
With this deeper point of origin may also perhaps be associated the develop- 
ment of the pseudoperidium, which would protect the young structure while 
pushing its way to the surface. 
There can be little doubt that the aecidium, when present, is always 
the seat of the process of transition from the condition of single to that 
of paired nuclei. Sapin-Trouffy (48) found this to be the case in all the 
forms (about fourteen) which he investigated. His observations have been 
confirmed by Maire (29 ; 30) for Endophyllum Sempervivi and for Puccinia 
Bunii , DC., and by the writer in the two forms here described, and also 
in Puccinia Poarum , Niels, P. car ids , Rebent, and in Uromyces Poae> Rabh. 
The conclusions as to the nature of the aecidium in Phragmidium must then 
apply throughout the group, and the aecidium in all cases must be con- 
sidered as a group of female reproductive organs 1 . Whether in all cases 
the transition (fertilization) is brought about by a nuclear migration, or 
whether in some cases the two nuclei are formed in the cell by division (as 
Maire believes), must be left to future work to decide. Such a condition 
would, however, be merely a further stage of reduction in fertilization, 
comparable to the well-known case of fertilization by means of a sister- 
nucleus (the second polar body) observed in the * parthenogenetic ’ eggs of 
Artemia. 
The hypothesis put forward by Poirault and Raciborski, that the 
sporidium may become binucleate and so start a mycelium with paired 
(conjugate) nuclei, is quite untenable. The work of Sapin-Trouffy, of Maire, 
and the observations here detailed have shown that in all cases investigated 
the mycelium which arises from the sporidium has single nuclei. It is true 
that in certain cases, such as in Coleosporium Euphrasiae , investigated by 
1 In Coleosporium what are usually known as the uredospores are produced in chains and have 
intercalary cells between them , and are thus liable to be taken for the true aecidiospores though no 
pseudoperidium is present. In forms such as C. Senecionis , Fr., in which the full life-history is 
known (about fifteen species of this genus are now known to be heteroecious), there can be no 
confusion between the two, as the true aecidium is found early in the year on the one host, is 
associated with spermogonia, and has a definite pseudoperidium ; while the aecidiospore-like uredo- 
spores are produced later, on the other host, and are intercalated, like ordinary uredospores, in the 
life-cycle between the true aecidiospores and the teleutospores. Sapin-Trouffy has shown, as was to 
be expected, that in C. Senecionis the transition from the single to the paired nuclei takes place in 
connexion with the true aecidium, while the aecidiospore-like uredospores are borne on a mycelium 
with paired nuclei derived by infection from the aecidiospores. Owing to their position in the life- 
cycle, the histological nature of the mycelium which bears them, and the absence of a pseudo- 
peridium, it is clear that the spores in question partake much more of the nature of uredospores than 
of aecidiospores, so the former name should be retained for them in spite of their development in 
chains. Bearing in mind the heteroecious nature of nearly all the forms of Coleosporium hitherto 
investigated the view put forward by Holden and Harper (23) can hardly be accepted, that the 
uredospores are really aecidiospores in the form to which they give the name C. Sonchi-arvensis , 
Lev., and of which they observed only the stage with uredospores and teleutospores. 
