202 MORPHOLOGY. 



the appearance of the seed-bud (ovule), within which alone the deve- 

 lopment of the appointed end of the pollen-tube into a new plant can 

 take place. Directing our attention to the universally occurring, and 

 consequently essential, characteristics presented by the seed-bud, namely, 

 the disproportionately great developement of one single cell (the em- 

 bryo-sac) within a cellular body free at one end and containing cyto- 

 blastema, out of which, at least in all the Phanerogamia, cellular tissue 

 is formed, there is clearly displayed an analogy, which can scarcely be 

 mistaken, with the antheridia of the Mosses and Liverworts ; and we 

 have in this structure an interesting example in the vegetable world, of 

 a fact often presenting itself in the animal kingdom, namely, a deter- 

 minate product of the formative force (a morphological organ) present 

 in one group, without possessing the same physiological importance which 

 it has in another group, and without the morphological organ becoming 

 a physiological organ. On the other hand, when we have conceived the 

 comparison in these two large groups, we may make the reasoning, evi- 

 dently deducible from the resemblance, of service as a safe point of de- 

 parture from which to arrive at further analogical conclusions. If we 

 have identified the pollen-grain and the spore, and if we have found 

 their development into new plants to be similar in the main points, we 

 may venture to expect similarity in the less important particulars. It is 

 now certain that in the Agamce one end of the tube of the spore (as in 

 the Ferns and Equisetacece) can, without the aid of any other organ, 

 develope new cells as the foundation of a new plant. Hence, I seek in 

 the Phanerogamia also the essential cause of the formation of new cells, 

 and consequently of an embryo, in the power of development of one end 

 of the pollen-tube, which is perhaps called forth and modified by the 

 action of the embryo-sac, but which appertains simply and exclusively 

 neither to this nor its contents. We do not by this means acquire 

 any results as to the nature of the sexual plants, yet we do gain a valu- 

 able leading principle to guide us in further researches, and in the 

 critical examination of the results obtained. Thus, the opinion which 

 I delivered on the formation of the embryo of the Phanerogamia 

 would have been justified, even if decided observation did not support 

 me, and if Meyen * had been right in asserting that the new cells 

 are formed externally on the apex of the pollen-tube (and not in the 

 inside of it, as I have seen). This mode of formation might occur in the 

 Agamce, as, for instance, Mirbel actually asserted in his work, already 

 referred to, on Marchantia, which researches I indeed must consider 

 very imperfect. On the other hand, the opinion that the first cells of the 

 embryo are not formed inside the embryo-sac, while the pollen-tube re- 

 mains outside it, is supported by the analogies to be drawn from the inves- 

 tigation of the Rhizocarpea, where the embryo is undoubtedly formed by 

 the end of the pollen-tube, outside and scarcely in immediate contact 

 with the embryo-sac ; leaving out of consideration the improbability 

 that, in this process, three forms so essentially different should be exhi- 

 bited, without even being respectively confined to definite groups, as 

 Meyen j" must allow from his own researches. 



114. All sexual plants possess stem and leaves, the latter at 



* Physiologic, vol. iii. p. 307, et seq. 



f Physiologie, vol. iii. p.. 307, 308., compared with 310, 311., and, still more de- 

 cidedly, 313. 



