ALTERNATION OF GENERATIONS. 227 
which the whole process of development is again repeated. And as in Ferns, in 
addition to the spores, bulbils are also sometimes developed on the mature plant, 
so penicilliate conidiophores are also occasionally produced on the tuber of 
Penicilliuvi, For the purpose of including this second stage of development in 
the various classes of plants under a common name, it may be termed the Spore- 
forming or Asexual Generation [sporophore], excluding from the idea of true 
spores the conidia of Penicillium, as well as the gonidia of Thallophytes. 
If now, in the three examples cited of alternation of generations, we compare 
the organisation of the first or sexual with that of the second or spore-forming 
generation, it is seen that the latter, the result of an act of sexual union, is more 
highly and perfectly developed than the former, and therefore in this respect 
also represents the true close of the process of development. Thus in Penicillium 
we find the first generation developed in the form of a so-called mycelium, 
consisting of slender segmented branched filaments, while the second generation 
consists of a compact tissue of complicated structure. In Mosses again the 
first generation commences with a protonema, consisting, like a mycelium, of 
branched segmented filaments of cells ; but here this generation advances to a 
higher development, since the protonema produces the leaf-bearing Moss-plant, 
the histological structure of which is still however very simple in comparison with 
the much more complete difi"erentiation of the sporogonium. Still more strikingly 
are these characteristics seen in Ferns, where the first generation or prothallium 
consists of a plate of tissue which shows scarcely any external differentiation, 
while the second generation or true Fern is a very highly organised plant, dif- 
ferentiated externally into root, stem, and leaves, the tissue itself being also 
differentiated into three well-marked systems, the epidermal system, the fibro- 
vascular bundles, and the iundamental tissue. 
Starting then from Algse and Fungi, and proceeding through the classes of 
Muscine3e,'Filices, and Equisetaceae to the Lycopodiaceae, and finally to the Phane- 
Togams, it is seen that in the alternation of generations, the first generation 
(oophore) continually recedes in importance and independence, while the develop- 
ment of the second generation (sporophore) continually advances ; so that at length 
in Phanerogams the former is no longer a plant with independent power of growth, 
but takes the form of a special mass of tissue, the so-called Endosperm in the repro- 
ductive apparatus of the latter, filling up along with the embryo the cavity of the 
seed-coats. In contrast to this, at the starting-point of the series (Algae and Fungi), 
the first or sexual generation is alone developed as a plant with independent growth ; 
the second (asexual) generation appearing on it as its fructification or spore-fruit, 
represented, in its simplest form, by a single spore resulting from fertilisation, as 
will be illustrated in the introduction to the Thallophytes. 
Designating the course of development which we have sketched out as 
alternation of generations, each of the two stages may be termed an Alternating 
Generation. Each may, as we have seen, be propagated directly by gemmae, or 
by conidia, those developed by the first generation again producing individuals 
of the same kind; and in the same manner bodies of the same nature produced 
by the second generation will reproduce it. But this mode of reproduction may be 
wanting in either of the two alternating generations. 
Q 2 . ^ 
