A natomico-physiologica l Relations in the Spermophyte Shoot. 157 
Fig. 32). This example is of particular interest in view of the strict 
comparison it affords with the monocotyledonous condition as exemplified 
in the Gramineae, Palmae, Commelinaceae, and other families. 
Monocotyledons . 
Tinantia fugax, Commelina coelestis, Trade scantia fluminensis, Zebrina 
pendula (Commelinaceae). The first three plants furnish perhaps as striking 
an illustration as can be found of the demarcation of the fused edges of the 
leaf by a line of hairs. In each of the genera the leaf-insertion embraces 
the axis. Above the node level the leaf forms a short tubular sheath, 
surrounding but free from the axis, before it expands into the free lamina, 
the basal margins of which are ciliate. On the side of the sheath which 
represents the fused edges a well-marked line of hairs 'is present, which, 
except in Zebrina pendida , is continued past the node and down the whole 
length of the internode beneath, until it terminates to one side of the 
mid-point in the axil of the next leaf below (see Fig. 33). Elsewhere the 
surface is glabrous except at the insertion level in Zebrina where hairs occur 
especially over the spot 1 where the axillary bud or a root will break 
through later. In the flowering region, as is so often the case also in 
Dicotyledons, the plant may become generally hairy (Tinantia). In the first 
two genera named above 2 there are no hairs on the well-developed hypocotyl, 
a fact which becomes comprehensible when it is seen that the margins of the 
cotyledon sheath, unlike those of the leaves, are without hairs. 
Gramineae. In the adult plant it needs no very close examination 
to see that the base of the split leaf-sheath is not the lower lirqit of the ex- 
ternal tissue of the leaf, and that the * skin ’ or outer surface, now fused with 
the axis, is continued down for one internode below the level of inser- 
tion ; while in regard to the morphological nature of the structures 
present in the seedling the view arrived at by Sargant and Arber, 3 as the 
result of a detailed study of this family, is not only entirely compatible with 
the position here taken up but affords it a certain measure of support, not- 
withstanding the additional complications which render the elucidation of 
the Gramineae particularly difficult. For it may well be that such a 
process of fusion as is envisaged in the present account preceded phylo- 
genetically the further fusion which these authors hold to have occurred 
through close juxtaposition of the cotyledon stalk with the hypocotyl 
and thus to have formed the ‘ mesocotyl \ 
Dioscorea quinqueloba . The leaf arrangement and insertion, and the 
resulting surface pattern, are so similar to the configuration seen in the 
1 Arising presumably as the direct outcome of pressure or strain set up by the underlying 
. protuberance. 
2 Seedlings of the two other types were not obtainable. 
3 Ann. Bot., vol. xxix, 1915. 
