The Morphology of Ruppia Maritima. 89 



in PI. VII, fig. 4.3 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, L vm- this protuberance soon difteren- 

 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 VII, 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 (PL 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 (PL I, fig. 2; 

 PL II, fig. 6). 



1. The Leaf Blade. 



A cross section of a leaf, made above the region of the sheaths, 

 is represented in PL VII, fig. 43, /i(^'. An epidermis of comparat- 

 ively narrow cells, a subepidermis of wider cells, a single axial vas- 

 cular bundle with one subsidiary bundle nmning 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. 



