732 
Thou tas. — Seedling Anatomy of 
identical in closely allied tetrarch and diarch forms, and has more in 
common in the majority of seedlings than has the anatomy of the cotyledons 
or of the roots (compare Magnolias and Liriodendron , Berberis sps.). The 
hypocotyl shows a varying number of primary centres of xylem which 
alternate with two phloem groups and from which differentiation proceeds 
in a more or less tangential direction on either side. These triads 1 recall 
the ‘ divergeant ’ of Bertrand and Cornaille , 2 and they seem to constitute 
the units upon which the vascular system of hypocotyl, cotyledons, and 
root (perhaps also that of the plumular leaves, example Draba) is built 
up. They may pass out into the leaves practically unaltered and down- 
wards into the root with very little change, but they may be very seriously 
modified by more or less unknown causes. The arrangement in the 
hypocotyl does not suggest to my mind root anatomy as it does apparently 
to M. Chauveaud, but rather a stele of a less specialized character more 
comparable to the structure of the stem axis of primitive cryptogams both 
living and extinct. Thus it has the essentials of the exarch to mesarch 
solid or hollow steles of the Osmundaceae, &c., and the leaf-traces passing 
off from it are also suggestive of those of the primitive forms. The three 
centres of xylem differentiation so often seen in the ‘ double bundle ’ of 
Phanerogams render this leaf-trace remarkably like the leaf-trace of 
Osmunda rcgalis (see Bertrand ), 3 if we neglect the internal phloem of 
the latter. 
Our knowledge of the more primitive Spermophytes, living and 
extinct, points to the probability of the ancestral Spermophyte having been 
a plant of robust organization with large seeds. 
This inclines one to the view that not only is the tetrarch condition 
probably relatively primitive, but that it is very possibly itself derived from 
a larger plan such as we see in some of the Pyrus forms, in Lauras nobilis , 
and in certain Amentiferae. The suggestive Cryptogamic characters of the 
hypocotyl are, however, independent of number and size, and are seen in the 
slenderest forms, the larger ones possessing a greater number of units which 
are the triads similar to the ‘ divergeants * of Bertrand and Cornaille . 4 
Several authors have decided that seedling anatomy is of no phylo- 
genetic importance, by which apparently they mean that it does not furnish 
sharp taxonomic characters. The phenomena upon which they have fixed 
their attention, namely, the broad features of transition and particularly 
the polarity of the root, are not, I think, likely to do so j but there are small 
characters indicative of relationship which become familiar to the worker, 
1 The presence of isolated protoxylem elements in the upper part of the hypocotyl of Caly- 
canthus, Ricinus , and other forms in which the hypocotyl appears to have a modern stem structure 
points to the derivation of this form from the structure described. 
2 Bertrand, E. C., and Cornaille, F. : La masse libero-ligneuse etementaire des Filicinees 
actuelles. Trav. et Mem. de l’Univ. de Lille, vol. lx, 1902. 
3 Bertrand, P. : Progressus, Bd. 4, Heft 2, 1912. 
4 Loc. cit. 
