The Morphology of Ruppia Maritima. 109 
A four-layered type has indeed been known to be unusual since 
De Bary’s (1884, p. 9) work, in which he describes it as appearing 
in only two plants, Pistia and Hydrocharis. Recently Campbell 
(1897, p. 40) has added Zannichellia to this list. 
Beyond the papers of Miss Scott and Campbell, I find no record 
of other investigation on the apical meristem of the root tips of the 
Potamogetonaceae, if we except Potamogeton crispus and natans, 
studied by Treub (1876) and Flahault (1878), who assign to them 
three meristematic layers. 
Considerable doubt and uncertainty have arisen concerning the 
tissue-forming capabilities of the various meristematic tissues of the 
growing point, a subject on which Scott (1894) has written an ad- 
mirable resume. Histogenesis in the roots of Ruppia is, however, 
perfectly clear. The plerome can be traced in its development into 
the central cylinder, communicating with the central cylinder of the 
stem. Thus, Pl. VIII, fig. 44 shows a plerome cylinder connecting 



ep=— -—-- 
Ss OOOO) 
Hy) 
Tee 
| DANTE, 
Figure 26.— Cells: from outer part of cross section of 
root ; ef, epidermis, ex, exodermis. Showing also two 
or three layers of cortex with intercellular spaces. >< 325. 
with the central cylinder of the stem. The width of this plerome 
at its base is about eight cells, which corresponds to the number 
of cells in the diameter of the central cylinder of the mature root— 
i. e., 7—10 (PI. VIII, fig. 46). 
3. The Anatomy of the Mature Root. 
The internal structure of the root is even simpler than that of the 
stem. It consists of an epidermal region and a large zone of corti- 
cal parenchyma limited on the inside by the endodermis which 
surrounds the axial vascular bundle (PI. VI, fig. 27). Each of these 
four parts will now be described in detail. 
a. Epidermal Region. 
The epidermis is a layer of large, thin-walled cells (Text-fig. 26), 
some of which produce the root hairs, to be described in detail pres- 
ently. Beneath the epidermis, but not connected with it genetically, 
