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 alfernation 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—drood-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. 
