82 MORPHOLOGICAL DEVELOPMENT. 



I 



grounded theory of the origin of the differentiated shoot- 

 system of the higher plant. I confess I don't think it can 

 be done at all with the materials at present at onr disposal. 

 Of course it is just possible to suppose that some ancestral 

 sporophyte had the structure of a proliferous thalloid liver- 

 wort gametophyte, and that from it was evolved the phanero- 

 gamic shoot in the ways you suggest. This gives us abso- 

 lutely no clue, however, to any Pteridophytic shoot, which 

 ought to be intermediate (more or less) between the hypo- 

 thetical ancestor and the Phanerogam, and is furthermore, 

 as far as I can see, not supported by an atom of evidence of 

 any kind. It is true that your theory fits in well with the 

 phenomena exhibited by phanerogamic shoots themselves, 

 but this fact you will see must lose much of its significance 

 if the hypothesis lacks foundation. 



" With regard to your method of explaining the funda- 

 mental characters of ' Exogens ' and ' Endogens/ this of 

 course is part of the same hypothesis; but I may point out 

 that since Yon Mohl and Sanio, between 1855 and 1865, 

 showed (1) that the growth at the stem apex of a mono- 

 cotyledon was not endogenous, and (2) that the ' thickening 

 ring J near the apex of a dicotyledon was not to be confused, 

 as had been done up till then, with the ring of secondary 

 meristem or true cambium, which arose lower down, and only 

 in woody or practically woody stem, the terms ' Exogen ' and 

 ' Endogen ' have necessarily fallen into disuse, since they 

 imply a false conception of what happens. Both monocotyl- 

 edons and dicotyledons have a i thickening ring/ which 

 gives rise to the primary vascular cylinder of the stem. 

 When the stem is of considerable thickness, as in Palms, &c, 

 it grows by the active cell-division of its outer layers, so that 

 both classes are f exogenous ' in this sense ; while the addition 

 of a centrifugal zone of secondary wood is confined to certain 

 Dicotyledons (Trees, shrubs, &c). 



" The distinction between the embryos, moreover, is not 

 absolute. The single cotyledon is usually terminal in mono- 



