228 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 spore? with which the development of the second generation closes, 

 we term all those reproductive organs which immediately propagate the same 

 generation either GemmcB 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 | 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. 



