124 DIVISION II. COURSE OF DEVELOPMENT OF FUNGI. 



generations of independently living bions proceeding alternately from one another. 

 This phenomenon has been known since Steenstrup's time as alternation of generations^. 

 This takes place in the Ferns between the sporophyte developed out of the 

 archegonium as the one generation, and the prothallium which grows from the spore 

 and bears archegonia as the other. The two chief stages in the course of the 

 development answer in this case to the two alternating generations, and hence 

 the term generation or alternating generation has been extended to the two relatively 

 homologous chief stages in the course of the development which were distinguished 

 above, and its rhythm as there described has been commonly termed the alternation 

 of generations, without regard to the biontic independence or continuity of the 

 successive stages. This is one meaning of the expression alternation of generations, 

 and it is the one strictly adhered to by Sachs especially (see his Text-book). By this 

 use of the expression the entire course of development is always composed of two 

 alternating generations, the sexual which closes with the production of sexual organs, 

 and the asexual which proceeds from the other and forms spores, or of the homologues 

 of these two generations. 



Further complications may make their appearance in the course of the develop- 

 ment which we are describing and at very different places in it, inasmuch as portions 

 besides the archicarp and its products may separate as reproductive organs from 

 the body in one stage of the development, and grow into new independent bions 

 resembling their immediate parent. Each of these bions may then under favourable 

 conditions reproduce the other stage of the development, the other alternating 

 generation, and thus return into the typical path. Organs of reproduction of 

 this kind increase the number of single segments of a stage of the development ; they 

 may be compared to branches and are in fact connected with them by many 

 intermediate forms, being generally distinguished from them only by the fact that they 

 separate from the parent-form, while what is termed a branch does not. They 

 usually serve in an especial manner for the multiplication and dissemination of the 

 bions belonging to the particular species, and are therefore fitly termed organs of 

 propagation. They are always asexually produced and separate in very different states 

 of development from the parent-shoot; they may be highly differentiated shoots, 

 or small tubers composed of a few cells, or single cells brood-buds, bulbils, brood-cells 

 (spores), &c. They are wanting in some species, as in Vaucheria aversa, V. dichotoma, 

 Preissia commutata, many Filices and Dentaria pinnata, while they occur abundantly 

 in their nearest allies, as for example in Vaucheria sessilis, V. sericea, Marchantia 

 polymorpha, Lunularia, some Filices and Dentaria bulbifera. In the latter case they 

 may not only become highly characteristic members of the species, but may actually 

 multiply the species through an unlimited number of generations and always in the 

 same form, and thus divert it from the typical rhythm of the development which 

 is preserved unaltered by the allied forms. External causes often cooperate 

 to a considerable extent in this process, and the possibility of a return to the typical 

 rhythm is still preserved as was stated above. It is easy to ascertain this rhythm in 

 the more highly differentiated forms, in the Phanerogams, Filices, Mosses, and some 

 Algae. To do so in the lower and comparatively simply organised plants requires 



Ueber d. Generationswechsel, Kopenhagen, 1842. 



