1058 REPORT— 1898. 



one, and now to the other. Vines (1877) held that most of the Thallophytes have no 

 alternation of generations at all, since their mode of propagation, whether sexual or 

 non-sexual, is directly dependent upon external conditions ; that a definite alter- 

 nation of generations, comparable to that of the Mosses, is only found in Coleochate 

 and Chara. Celakovsky (1877), however, wasmore in accordance with Pringsheim 

 in his conception of the Thallophytes, for like him he accepts an alternation of 

 neutral and sexual generations. Celakovsky designates this alternation of genera- 

 tions as homologous, since the successive generations are equivalent to one 

 another. Celakovsky opposes Pringsheim in his conception of the alternation of 

 generations in tlie Archegoniatte, which he designates as the antithetic. Here the 

 two alternating generations are not homologous, but essentially different ; the 

 non-sexual generation has also phylogenetically nothing to do with the neutral 

 generations of the Thallophytes. This conception of Celakovsky was at first 

 neglected, but was taken up again by Bower (18'JO), and put on a footing of 

 detail. Bower holds that tlie antithetic alternation came about by the interca- 

 lation of the non-sexual generation, the sporophyte, as a quite new development 

 between two gametophytes. This interpolation of a .special sporophyte probably 

 took place in the alga-like ancestors of the Archegoniatfe, as they passed from a 

 life in water to a life upon the land. In the series of the Thallophytes there are, 

 in addition to the homologous alternation of generations, more or less advanced 

 beginnings of an antithetic alternation, as for instance in Coleoehcete, the Ascomy- 

 cetes, and Floridete. 



All the various conceptions of alternation of generations in the Thallophytes 

 rest on morphological comparison of the hitherto known facts of the life history, 

 while still very little was known of the behaviour of these organisms in open 

 nature, or in long continued cultures. Yet by such observations only is it possible 

 to decide whether an alternation of generations can be proved at all, and how the 

 influence of the outer world, so often assumed, really affects the course of life of the 

 Thallophytes. These questions were the point of departure for my investigations 

 on the conditions of propagation in the Thallophytes. The investigation had to be 

 extended in two directions : in the first place, it was necessary to decide whether 

 there is a regular alternation of free and independent generations ; secondly, 

 whether a non-sexual generation characterised by special qualities arises of inner 

 necessity from the fertilised ovum. 



The first question receives its answer, according to the results of my investi- 

 gations, that no regular alternation of neutral and sexual generations exists in any 

 of the Thallophytes which have been tested. They possess two or several kinds of 

 propagation, each of which is directly dependent upon quite definite external con- 

 ditions. If we take any vegetative stage we please, a filament of a Vaucheria, an 

 (Edoffonium, a piece of mycelium of a Spovodinia or Ascoidea, there are then 

 present in each part the specific potentialities of sexual and non-sexual propa- 

 gation. In open nature the fortuitous conditions determine which of the poten- 

 tialities is developed, and how the modes of propagation follow one another, 

 whether upon the same individual, or on diSerent individuals. An exact know- 

 ledge of the conditions gives the experimenter the secure control over the organism, 

 which can at will be forced into any desired mode of propagation within the limits 

 of its species. The problem becomes more complex if other additional modes of 

 propagation appear, as they do in many Fungi. In Saprolegnia we can distinguish 

 four kinds of propagation : (1) by simple mycelial growth, that is, by breaking of 

 the mycelium into pieces ; (2) by zoospores ; (3) by oospores ; (4) by geoimse. 

 The conditions for each of these four kinds of propagation are somewhat different, 

 and it is thus possible, as my later investigations prove, to force the fungus at will 

 into one or other of the four modes of propagation. 



The potentialities for the several kinds of propagation in the Thallophytes, 

 such as Vaucheria, (Edogonium, Chlamijdomonas, Uporodima, Saprolcpiia, Eurothmi, 

 Sec, are quite equivalent, i.e. there is no cause in the inner nature of the cell, or in 

 the special organisation of the potentiality, for one of these of its own initiative 

 being developed earlier or later ; nevertheless, according to the species, the special 

 conditions which dominate the kinds of propagation may be more or less readily 



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