Notes . 
58i 
asciis 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 fertilized ovum, e. g. in Sphcierotheca 
according to Harper, or the Laboulbeniaceae according to Thaxter. 
In others we may regard a structure homologous with the ovum, but 
not actually capable of fertilization, 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 Pyrenomycetes, 
and the Cladonias among the Lichens, &c., the fruit is, according 
to our present knowledge, exclusively a product of the mother-mycelium, 
just 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 Asco- 
mycetes it is still more clearly to be recognized 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attempts, and in the higher forms has been replaced by direct develop- 
ment of the fruit from the mycelium. As regards the solution of the 
question how the alternation of generations in the Archegoniatae 
came into existence, the Ascomycetes can contribute far less than the 
Florideae. 
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 appearance of an alternation of generations : — 
1. The majority of the Algae and Fungi have two or more kinds 
of propagation, 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 fertilized ovum in sexual 
forms does not differ essentially on germination from another propa- 
gative cell. In none of these cases is there any reason for speaking 
of an alternation of generations. ~ 
2. In certain heteroecismal parasites, e. g. many Uredineae, 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 characterized by different 
