582 ALEXANDER W. EVANS 



and Williams' figure of P. Zollmgeri (Gottsche) Schiffn.^ It is found not only 

 in the genus Pallavicinia but also in the related genera Syviphyogyiia and 

 HynicnopJiytum. t 



The other tissues of the rhizome are likewise continuous with the midrib 

 of the winged thallus. This midrib is distinctly convex ventrally and slightly 

 so dorsally. It is bounded on each surface (Fig. 6, A) by a layer of green 

 cells with bulging and slightly thickened walls, averaging about 13 a in width. 

 The cells between these surface-layers and the median .strand are usually in 

 five or six layers and average about 26 a in diameter. They are pale or 

 colorless and store starch in greater or less abundance, and their walls are either 

 very thin throughout or slightly thickened in the outer layers. 



In typical cases the midrib merges very gradually into the wings, although 

 the transition may be more abrupt. The unistratose portions of the wings vary 

 in consequence from a width of only two or three cells (Fig. 6, C) to a width 

 of twelve to fifteen cells, and marked diff"erences in width are often to be 

 observed in different parts of a single thallus (Cf. Fig. 6, D, with Fig. 6, C.) 

 In cases where the transition is gradual, the internal cells in the transition- 

 region are often distinguished by their large size (Fig. 6, B) and may attain 

 a width of 60 ;j. or more. These cells show very conspicuous!}- in preparations 

 cleared with potash, and the marked contrast in size between them and the 

 surface cells can be easily demonstrated by focussing. Sometimes, however, 

 as shown in Fig. 6, C, the distinction between superficial and internal cells tends 

 to break down. 



The cells in the unistratose portion of the wings are variable in size but 

 average about 30 a in diameter. An occasional marginal cell may be longer 

 and narrower, measuring perhaps 40x15 [J-, but cells of this type are so in- 

 constant that they have but little taxonomic significance. The walls of the alar 

 cells are either thin or slightly and uniformly thickened. At fairly regular in- 

 tervals along the margin the wings develop characteristic appendages in the 

 form of slender teeth (Fig. 6, E — I). These teeth are mostly three or four cells 

 long and two to four cells wide at the base. Except in the lower part, where 

 a gradual transition into the unistratose wings is to be observed, the cells of 

 the teeth are distinguished by their size, a length of 90 — 120 jx not being un- 

 common. The walls of the elongate cells are usually more or less thickened, 

 although this feature it not shown in the figures. The teeth often lie parallel 

 with the wing-margin or spread slightly, but they are sometimes curved inward 

 and may be more or less appressed to the dorsal or ventral surface of the 

 wings. 



GOEBEL^ has shown that the marginal teeth of P. xiplwides develop in 

 very young segments and that they arch over the apical region and may even 

 be more or less interwoven, thus forming a protective arrangement. This is 

 supplemented by the abundant development of two-celled slime-papillae on both 

 surfaces of the thallus. It should be noted, however, that the teeth themselves 

 do not secrete slime and that none of the papillae are marginal in position. 



* Morph. study of some members oi the genus Pallai>iti?iia fig. 2, B (1914). 

 ^ Flora 96: 184. f. 137 (1906). 



