TRANSACTIONS OF SECTION K. 1063 



the mother-plant. Beyond this no one will wish to assert a nearer relation of 

 kinship between Mosses and Floride?e. 



It is still more difficult in the Ascomycetes to decide the question of the alter- 

 nation of generations than in the Florideje. Notwithstanding the remarkable 

 differences which are to be observed in them up to the origin of the ascus fruit, 

 we must still, with De Bary, regard the whole group as a single and united one. 

 But we ought not to connect the Ascomycetes with the Phycomycetes, either with 

 the Peronosporese after De Bary, or with the Mucorineae after Brel'eld ; but we 

 should recognise in them a group which, with its simplest forms, sends out its 

 roots into the lowest division, the Archimycetes, to which the Chytridese and 

 other Fungi belong. In quite simple Ascomycetes, e.g. Ascoidea, Dipodascus, 

 Endomyces, there appears a striking difference in the mode of origin of the asci. 

 In Ascoidea and Endomyces, according to Brefeld, each ascus arises directly from 

 a mother-cell of the mycelium. In the nearly related Dipodascus, according to 

 Lagerheim, two cells coalesce, and it is the product which grows on into the 

 ascus. In the one case — apparently the more common one — the ascus is a direct 

 product of the mother-plant ; in the other forms we may speak of the beginning of 

 a non-sexual generation, if we pass on to the fruit-bearing forms in some still 

 relatively simple species we find the asci as products of a fertilised ovum, e.g. in 

 Sphcprotheca according to Harper, or the Laboulbeniacese according to Thaxter. 

 In others we may regard a structure homologous with the ovum, but not actually 

 capable of fertilisation, as the starting-point for the formation of asci, while other 

 constituent parts of the fruit, such as the wall, and commonly the stalk, &c., are 

 supplied by the mother-plant. In the highest forms the most complicated Pyreno- 

 mycetes, and the Cladonias among the Lichens, &c., the fruit is, according to our 

 present knowledge, exclusively a product of the mother mycelium, j ust as is the 

 case according to Brefeld in the Basidiomycetes. It is only in case of the simpler 

 forms that we can compare the ascus fruit with the sporogonium of the Mosses, 

 and, as Oltmanns has done, place it in relation to the processes in the Florideae. 

 In the Ascomycetes it is still more clearly to be recognised than in these plants 

 that the antithetic alternation of generations has stood still at the first attempts, 

 and in the higher forms has been replaced by direct development of the fruit from 

 the Mycelium. As regards the solution of the question how the alternation of 

 generations in the Archegoniatte came into existence the Ascomycetes can contri- 

 bute far less than the Floridete. 



Taking a general view of the department of the Thallophytes thus traversed, 

 the following cases may be distinguished as relating to the question of the appear- 

 ance of an alternation of generations : — • 



1. The majority of the Algae and Fungi have two or more kinds of propa- 

 gation, each of which necessarily depends upon definite external conditions 

 characteristic for it. According to the conditions, occurring fortuitously in open 

 nature or in cultivation, the kinds of propagation may appear on the same or on 

 different individuals, independently or in any succession. The fertilised ovum iu 

 sexual forms does not difier essentially on germination from another propagative 

 cell. In none of these cases is there any reason for speaking of an alternation of 

 generations. 



2. In certain hetersecismal parasites, e.g., many Uredinese. the life-history of 

 the species takes the course of a regular succession of different individuals with 

 special modes of propagation ; we may here speak of an alternation of several 

 generations characterised by different propagation. There would be in this case 

 an alternation of homologous generations in the sense of Celakovsky and Bower. 

 Here, also, we have essentially to deal only with Fungi which have dimorphic or 

 polymorphic propagation, with the limitation that the external conditions for some 

 of the forms of propagation are so different that, so far as experience yet goes 

 these are only developed upon separate host-plants. 



3. In the unicellular Diatoms there is, according to the theory of the cause of 

 auxospore formation hitherto current, an alternation of generations in the sense 

 that, after a definite number of cell-generations derived by division, the formation 



