Presidential Address to Section K. 
349 
homologous with it in the sense in which the earliest vascular 
formations in cotyledon and hypocotyl respectively were homologous 
with each other. He considers that the successive vascular 
formations—marked by the appearance of alternate, intermediate, 
and superposed xylem in turn—represent three successive phases of 
stelar development: the root stele corresponds with the first of 
these phases only. This implies the hypothesis that at some past 
period a group of plants in the direct line of descent of Angiosperms 
possessed a stele resembling that which is now a mere stage in the 
life of the individual; thus the alternate formation found throughout 
the very young seedling implies an ancestral group with an exarch 
stele in stem as well as root, and a leaf-trace of corresponding 
structure. If this view be adopted, the seedling must, during the 
period when it consists only of cotyledons, hypocotyl, and primary 
root, with the plumule present as a mere bud, represent a past 
period in race history when its ancestors possessed an exarch stele 
in both stem and root, when the stem stele belonged to the stem 
only and the insertion of leaf-traces hardly modified its structure, 
and when it entered the root without change, and therefore no 
transitional region occupied and puzzled the anatomist of the 
period! This early stage in the development of the seedling is 
succeeded by that in which the epicotyl (plumule axis) begins to 
grow, and as a rule the epicotyl is undoubtedly modern : its vascular 
skeleton is built up of leaf-traces which are endarch from the first, 
at the cotyledonary node they are inserted on the vascular cylinder 
of the hypocotyl which has become endarch at the top. This 
transition has been effected lower down in the hypocotyl, as 
described already, by the formation first of intermediate and then 
of superposed xylem together with the gradual disappearance of 
the original alternate xylem. Hence the cotyledonary node may 
be considered to mark the interval between two acts in the drama 
of evolution—an interval the length of which cannot be estimated, 
but is clearly to be reckoned in geological epochs. The race 
history of the phanerogamic stem-cylinder is at present unknown ; 
possibly the development of the hypocotyl may give a clue as 
suggested by Chauveaud, or Jeffrey may be right in deriving the 
leaf-traces from a simple tubular stele (siphonostele) which has 
become more and more broken up by the appearance of foliar gaps. 
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 should anatomists acknowledge that it cannot be 
considered as strictly homologous with the stele of the root, but 
much confusion of thought would be avoided if the two structures 
were not treated as strictly comparable. 
Apart from the foregoing consideration of 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, 
the evidence of embryology is of great weight in questions of 
internal morphology and phylogeny. Hanstein’s account of the 
Monocotyledon embryo suggests two distinct problems: (1) whether 
a terminal member can be considered as a leaf, (2) whether 
Dicotyledons are derived from a monocotyledonous ancestor, or 
Monocotyledons from a dicotyledonous form. The most obvious 
