88 



A. H. Graves, 



which I have termed the stipular sheaths (PL I, fig. 2 ; PI. II, 

 fig. 6). In external appearance the leaves very closely resemble 

 those of Potamogeton filiformis and Potamogeton pectinatus, both 

 also typical submerged plants fMorong, 1893). The leaves of 

 Zannichellia and Althenia are also macroscopically very similar: 

 those of all species of Naias are much shorter with strongly serrate 

 or toothed margins.^ 



Development of the Foliage Leaf Different stages of leaf 

 development are represented in PL I, fig. 1 ; PL VI, fig. 25, lon- 

 gitudinal sections of a vegetative bud, where the youngest leaf 

 is the protuberance at one side of the growing point (L^^^^ 

 in PL I, fig. 1). The next older leaf primordium, shown on 

 the right of the growing point (PL I, fig. 1, L^^^), has already 

 /^ slightly elongated, until now it extends 

 beyond the growing point. Successively 

 older stages are represented by Z.^-^, 

 LV, Li^, &c. 



PL VII, fig. 43 represents a transverse 

 section through a similar bud cut through 

 about the region of the line ab in PL I, 

 fig. 1. Therefore, we find here the different 

 members in exactly the same orientation 

 as in PL I, fig. 1. Thus, the outermost leaf, 

 L^, embraces with its sheaths all the 

 internal complex, just as is shown by Z,-'^ 

 with its sheaths in PL I, fig. 1. The next 

 inner member is /i'^), or the lowest and 

 largest foliage leaf of the axillary bud of 



Figure ll.-The marginal ^j ^^ien a section through the apical 

 leat teeth, snowing variation . . , , , . ^ . , \ 



in cell arrangement, x 300. portion of the scale leaf which envelops 



this axillary bud; next the leaf sheaths of 



the next upper main leaf, L-'-^, appear, which brings us to a cross 



section of the central stem: without going into more detail it is 



sufficient to note that the remaining parts may be correlated to their 



corresponding longitudinal sections in the same way. The line a b 



^ All these species belong to Hansgirg's (1903) " Vallisneria-Typns 

 der Stromungsblatter," a physiological group showing little differentiation 

 into blade and petiole and characterized by the lack of cuticle, hair 

 structures and stomata, by the absence of any considerable amount of 

 supporting tissue, and by the ribbon-like, isolateral form. 



