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 nutans, 

 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- 



Figui'e 26. — Cells from outer part of cross section of 

 root ; ep. epidermis, ex. exoderm.is. Showing also two 

 or three layers of cortex^vith intercellular spaces. x325. 



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. Vm, 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,. 



