702 TRANSACTIONS OF SECTION K. 
gaps. This process is said to be exhibited in the young epicotyl.’ Until this 
point is cleared up the exact relationship of the vascular cylinder of the stem 
to that of the root will remain obscure. As a matter of convenience the stem- 
cylinder will, no doubt, be called a stele, even though anatomists should acknow- 
ledge that it cannot be considered as strictly homologous with the stele of the 
root. Much confusion of thought would, however, be avoided if the two 
structures were not treated as strictly comparable. 
There can be very little doubt that the insertion of leaves has brought about 
the change, and I might suggest here that the insertion of leaves on an exarch 
stem-stele would be an interesting subject for research. The literature of the 
subject is scattered, and its treatment seems to me very incomplete. An exarch 
axis bearing leaves is, of course, exceptional, but more common among extinct 
plants than among recent species. So far as my very cursory examination of the 
literature has gone, it seems a general rule that the leaf-traces are inserted on the 
xylem poles of the stele.® 
Hitherto I have considered modern embryology in relation to a single problem 
of internal anatomy—namely, the comparison of the vascular system of the stem 
to that of the root. But the evidence of embryology is also of great weight in 
questions of internal morphology and phylogeny. 
Several questions of this kind are discussed by Hanstein, from whose classical 
paper I continue to date. For example, his account of the embryo of Mono- 
cotyledons suggests two distinct problems. One belongs to formal morphology— 
namely, the question whether a terminal member can be considered as a leaf. 
The other is a question of phylogeny : whether Dicotyledons are derived from a 
monocotylous ancestor or Monocotyledons from a dicotylous form. Both these 
questions I have discussed elsewhere, and only refer to them now as examples 
of the way in which seedling anatomy has proved complementary to that of the 
older embryologists. 
The most obvious interpretation of Hanstein’s observations is that the single 
cotyledon of Monocotyledons is equivalent to the pair found in Dicotyledons. 
This would imply that Dicotyledons were derived from an ancestor with one 
cotyledon, apparently terminal, which gave rise to the existing pair by a process 
of fission. But other interpretations were always possible, and the terminal 
hypothesis received a shock when Count Sohms-Laubach discovered that in 
certain Monocotyledons the single cotyledon is lateral from the first. 
The comparative antiquity of Monocotyledons and Dicotyledons has been 
one of the first questions raised by the study of seedling anatomy. It is remark- 
able that both the hypotheses founded on work of this kind assert the greater 
antiquity of the dicotylous form. But if the cotyledonary member of Mono- 
cotyledons is derived from one or both cotyledons of an ancestral pair, it cannot 
be considered as terminal. Thus the evidence of seedling anatomy bids fair to 
settle both these problems, as I think it will settle others of the same kind 
mentioned by Hanstein. 
The descriptive work of Irmisch and the school he represents has been carried 
on of late years by an American naturalist, Mr. Theo. Holm, with all the 
technical advantages given by modern instruments of research. His papers are 
commonly written with systematic intention, but the external characters of the 
species he describes are correlated with their internal anatomy, and the structure 
of the adult form is traced from its origin in the seedling. His monograph on 
Podophyllum peltatum is an example of this method, and illustrates its advan- 
tages in a very striking way. But it is becoming much more usual to compare 
the seedling with the adult form, as may be seen in two monumental works now 
being published in parts : ‘Das Pflanzenreich,’ edited by Engler, and ‘ Lebens- 
" Jeffrey, The Morphology of the Central Cylinder in the Angiosperms. 
Trans. Canadian Inst., vi., 1900. 
* D. H. Scott, Studies in Fossil Botany, 1908, p. 97 (Sphenophyllum); C. E. 
Bertrand, Remarques sur le ‘ Lepidodendron Harcourtii,’ 1891, p. 109; M. Hove- 
lacque, Recherches sur le ‘Lepidodendron selaginoides,’ 1892, p. 150; F. O. 
Bower, Origin of a Land Flora, p. 334 (Selaginella), 1908; C. E. Jones, Trans. 
Linn. Soc., ser. 2, vii., 1905, p. 19 (Lycopodium). 
* K. Sargant, Ann. of Bot. xvii., p. 1, 1903, and id. xxii., pp. 150-2, 1908. 
* 
“the 
