148 Saunders . — The Leaf-skin Theory of the Stem: 
of Dicotyledons especially, Chauveaud 1 has shown that the transition from 
the alternate to the superposed arrangement may take place at any level in 
the hypocotyl. He further concludes from the appearances at successive 
levels that this transition results from the gradual disappearance, as we pro- 
ceed up the hypocotyl, of the vascular elements of the root and their replace- 
ment on the alternate radii by the vascular elements of the cotyledon traces, 
from which observation he is led to the view that the superposed and alternate 
arrangements are characteristic, not of different morphological members, but 
of different phases of evolution. In a later paper 2 he formulates his idea 
of the construction of the plant body. He conceives it as composed of 
a succession of similar systems or units, each termed a f phyllorhize con- 
sisting of a leaf and a root, and each forming a bud from which is developed 
the succeeding ‘ phyllorhize \ These careful observations on the course 
of the vascular elements in the hypocotyl are of considerable interest, quite 
apart from this conception of evolutionary morphology which one may not 
be disposed so readily to accept. On the view that the existence of a stem 
axis has not yet been disproved, the question whether there is continuity 
or merely contact between the vascular elements of the trace and the root is 
one of prime importance. In the former case it may be urged that the 
region of transition cannot properly be referred either to the stem or the 
root ; in the latter case the lower limit of the stem tissue would be defined by 
the interdigitating ends of the vascular strands, the foliar character of the 
external tissue being unaffected on either view. This latter point being the 
one with which we are here mainly concerned, it may now be well to 
describe the topographical features of a few illustrative cases in further 
detail. 
4. Further Details of Surface Topography in some Illus- 
trative Cases among Angiosperms and Gymnosperms. 
Dicotyledons. 
Hypericum calycinum. Leaves opposite. A longitudinal welt marking 
the line of junction of the confluent leaf extensions runs down either side of 
the internode, starting from the points where the leaf-blades meet and 
terminating in mid-axil at the next node below. 
Epilobium parviflorum. Leaves opposite. Cotyledons glabrous. 
Hypocotyl glabrous, with a longitudinal welt on either side extending down 
from the points where the cotyledon blades coalesce. In the internodes of 
the young stem the raised contours representing the lines of fusion may be 
further marked by a distinct line of hairs (see Fig. 21). 
Ruellia amoena (Acanthaceae). Leaves opposite. Cotyledons with 
a short pile on the upper surface of the petioles, otherwise glabrous. 
Hypocotyl and stem with two well-marked lines of hairs extending through- 
1 Ann. des Sci. Nat., Bot., t. xiii, 191 1. 2 Comptes rendus, t. clviii, p. 343, 1914. 
