238 
MORPHOLOGY OF MEMBERS. 
If we now regard the two alternating generations as two stages of development 
of the same plant, each of which is necessary to supplement the other, it is seen 
that, in the first place, the entire course of development of a plant commences 
twice with a simple cell: — the first time the development begins with the spore 
to form the first (sexual) generation, the second time with the oosphere in the 
female organ to produce the second (spore-forming or asexual) generation. Secondly, 
we find that, in addition to these two beginnings from spore and oosphere, which 
are united to one another by the complete course of development, a subsidiary 
mode of development may also occur, each of the two generations having the 
power of propagating itself directly. For the purpose of distinguishing them 
from the true spores with which the development of the second generation closes, 
we term all those reproductive organs which immediately propagate the same 
genenation either Gemmce or Gonidia. A Spore, in our sense of the term, arises 
only from the second generation, and gives rise, on germination, to the first 
generation; a bulbil or gonidium, on the contrary, may arise from either of 
the alternating generations and reproduce it. The same facts may be ex- 
pressed in the following manner: — Sexual cells (oospheres) and true j spores 
indicate the turning-points in the alternation of generations ; they are not organs 
for direct reproduction, for each of them always produces something different from 
that from which it immediately sprung ; the spore of the Fern, for example, gives 
rise to a prothallium, the oosphere of the prothallium to a Fern; the spore of 
the tuber of Penicillium does not again give rise to a tuber, but to a fila- 
mentous mycelium, on which the tuber again arises as the result of fertilisation 
of the female cell. Bulbils and gonidia are, on the contrary, organs for direct 
reproduction, by means of which the same stage in the process of development 
is again repeated; the bulbil, for example, which arises on the leaf of a Fern, 
does not produce a prothallium, but a Fern; in the same manner the conidia 
formed on the branches of the mycelium of Penicillium do not, on development, 
give rise to the tuber, but to a mycelium like that on which they were borne. 
The alternation of generations, as we have now described it in a few examples 
where it is peculiarly well exhibited, does not occur in those classes of Thallophytes 
which have the simplest structure; its first indications are met with where an act of 
sexual union is first detected, until at length, in the more highly developed plants, 
the alternation is manifested with perfect sharpness. 
a. The idea of an alternation of generations is extended by some botanists con- 
siderably beyond the limits to which we have here confined it. It has been proposed, 
for instance, to apply the term to the case of Phanerogams in which lateral branches 
with foliage-leaves spring from a rhizome clothed only with scales, and from these 
other branches which develope into flowers; and to others of a like nature. It is 
clear, however, that the cases in question have a totally different significance in the 
history of development to that of the alternation of generations, using the term in 
the sense indicated above ; they might be included, by way of distinction, under the 
common phrase Alternation of Axes. This phenomenon is one which is very incon- 
stant even within limited groups of plants, while, on the other hand, true alternation 
of generations prevails over almost the entire vegetable kingdom; and the mode in 
which it runs through particular groups of plants is one of the weightiest arguments 
in favour of the natural system. This will be further elucidated in Book II, 
