8oO LECTURE XLVI. 



sexual organs become more and more developed, and the sexual reproduction 

 gains more and more over the vegetative, or even becomes the predominating 

 one. It would be possible to believe that with increasing perfection of the 

 organisation a corresponding division of physiological labour also is given, by 

 which the vegetative mode of reproduction is limited and the sexual one simply 

 promoted. 



I may finally notice yet another observation, which I have already expressed in 

 my 'Textbook' (Ed. IV, p. 877). Those Cryptogams which possess pronounced 

 alternation of generations, particularly the Mosses and Vascular Cryptogams, although 

 in other respects belonging to entirely different types, nevertheless illustrate repeatedly 

 and in every class the fact that the highest development of the organisation is always 

 attained only by means of fertilisation. In the case of the Equisetums, Ferns and 

 Lycopodiacese this is at once obvious on remembering that the first generation pro- 

 duced from the asexual spore, the prothallium, is usually a minute, very simple, and 

 transitory structure, the life and importance of which are closed with the fertilisation 

 of the oosphere ; whereas the embryo, originating from this act of fertilisation, de- 

 velopes into a highly organised plant, a Tree-fern, for instance, &c. In the case of 

 the Mosses, it is true, matters are diff'erent, in as far as here it is the generation which 

 has originated from the asexual spore itself which we are in the habit of regarding as 

 the proper plant ; but on comparing the forms of cells and tissues and the histological 

 structure generally of the 'Moss-fruit' produced by fertilisation with that of the Moss- 

 plant, there can be no doubt that the former is more perfectly organised than the 

 asexually produced Moss-plant itself. In the case of the Algse and Fungi it is the 

 same, although the fact is not so easily made out as in these cases. In those Fungi 

 in which a sexual apparatus is known, the most perfect product of the whole de- 

 velopment proceeds from the fertilisation. The asexual spore of the Ascomycetes 

 gives rise to the very simply organised mycelium, and it is only after the act of fertili- 

 sation on this that the Fungus-fructification with its complicated structure and high 

 organisation arises (see Fig. 409, p. 726), and similarly may be said of many Algae ; 

 even in the case where the result of fertilisation is only a single cell, an oospore or 

 zygospore (zygote), this tends to exhibit, at least in the development of its wall-layers, 

 a more perfect organisation than the vegetative parts of the same plant. 



Moreover the Phanerogams or Spermaphytes only apparently contradict the 

 above observation; in reality they confirm it in the most astounding manner. 

 As we have seen, the embryo-sac in the ovule is the true macrospore, in which the 

 first structure to arise is what is termed the prothallium in the Vascular Cryptogams : 

 the endosperm is a physiologically and histologically reduced and degenerated pro- 

 thallium, of which, strictly speaking, only the oosphere and synergidae survive in the 

 Angiosperms. What we designate generally the plant among the Spermaphyta is the 

 sexually-produced product, which also here has sprung from the fertilised oosphere, 

 while the preceding stage of development, corresponding to the prothallium, no longer 

 exists at all as an independent organism. 



In conclusion I may return to the phenomena of Apogamy already referred to. 

 With this word De Bary distinguishes the case, partly discovered by himself, where 

 asexual reproduction occurs in the place of sexual reproduction : this may happen in 

 very different ways — for example, by oospheres, which under normal conditions 



