The Morphology of Ruppia Maritima. 89 
in Pl. VII, fig. 48 represents the approximate plane in which a 
section similar to that in Pl. I, fig. 1 and Pl. VI, fig. 25 would 
be cut. 
With the help of these sections and other similar serial preparations, 
I have found that the course of development and distribution of 
growth in the ordinary foliage leaf is approximately as follows: the 
leaf primordium first appears as a mere swelling at one side of the 
growing point, Pl. I, fig. 1, 2/7; this protuberance soon differen- 
tiates into two parts, an “upper leaf,’ elongated in the direction of 
the growth of the shoot, and a “leaf base,’—-to use the terms of 
Eichler, 1865—consisting of lateral protuberances on each side of 
the base of the “upper leaf’ and extending part way around the 
shoot axis (Pl. VI, fig. 39). This arrangement is also shown, though 
not very clearly, by Pl. VI, fig. 28. The “upper leaf” will produce 
the leaf blade and the “leaf base” the leaf sheaths. The upper 
leaf now elongates rapidly, being composed entirely of embryonic 
tissue, and the succeeding stages are essentially those described by 
Prantl (1883): the cells at the apex are the first to commence 
extension to their mature size, and this stretching gradually proceeds 
toward the base of the leaf. The final developmental stage is 
marked by a considerable growth of the sheaths due to the inter- 
calary growth of the leaf, which Goebel (1898, p. 518) states is so 
characteristic of monocotyledonous leaves. 
A point of interest here is the comparative large size of the sheath 
rudiments at the first segmentation of the leaf primordium (PI. VII, 
fig. 39), a circumstance which will be discussed below under the 
heading of the leaf sheath. 
For purposes of further description, it is best to divide the leaf 
into two parts,—the blade and the sheaths, assuming the blade to 
be that part of the leaf from the sheaths to the apex (PI. I, fig. 2; 
Pl. Il, fig. 6). 
1. The Leaf Blade. 
A cross section of a leaf, made above the region of the sheaths, 
is represented in Pl. VI, fig. 43, 44). An epidermis of comparat- 
ively narrow cells, a subepidermis of wider cells, a single axial vas- 
cular bundle with one subsidiary bundle running along each leaf 
margin, a lacuna or air space on each side of the axial bundle, and 
a few extra interior layers of parenchyma cells toward both edges 
of the leaf and surrounding the vascular bundle comprise, in brief, 
the internal structure of the leaf blade. 
