THE TRACHEAL ELEMENTS IN THE FERNS

THE EARLIER botanists used the term "vessels" rather loosely as referring in general to the conducting elements of the vascular tissue, but it seems clear that in some cases at least the elements referred to as vessels were typical tracheids. Such for example are the "Treppengefiisse" of Weiss (1878) illustrating the con.ducting elements of Lycopodium. The tracheal elements of the Ferns are referred to by most writers as tracheids, although GwynneVaughan (1908) concludes that the tracheal elements in many ferns are vessels. The present investigation was undertaken to examine in detail the tracheal elements of representative types of the eusporangiate and the leptosporangiate ferns as classified by Eames (1936). These types include twenty-four genera of eight families as follows: Botrychium of the Ophioglossaceae, Angiopteris of the Marattiaceae, Osmunda and Todea of the Osmundaceae, Gleichenia of the Gleicheniaceae, Lygodium of the Schizaeaceae, Alsophila and Cyathea of the Cyatheaceae, Cibotium and Dicksonia of the Dicksoniaceae, and fourteen genera of the Polypodiaceaenamely, Adiantum, Asplenium, Blechnum, Davallia, Drynaria, Dryopteris (Nephrodium), Nephrolepis, Onoclea, Pellaea, Polypodium, Pteridium, Pteris, Thelypteris, and Woodwardia. True vessels are characterized by specialization in the terminal portion of the element and this specialization is expressed in the development of definite, usually large, openings of various kinds and usually also by the angular orientation of the end walls with respect to the side walls. Interpreted in harmony with this definition of the vessel, true vessels exist in Pteridium latiusculum (Desv.) Maxon (Pteris aquilina L.). The first statement of this fact as far as I can determine was made by Russow in 1873. Sachs (1874), DeBary (1884), and Strasbuirger (1891) figure these vessels in several aspects, and the observations of these early anatomists have been confirmed by Jeffrey (1917). Figure 1 shows the vessel element in Pteridium latiusculum isolated from macerated material of the stem. The side wall facing the observer is marked by scalariform pits through which the pitting of the opposite side, irregular because of contact with parenchyma cells, is visible. The end walls are clearly at definite angles with the side walls. Figure 2 shows a higher magnification of the terminal portion of a vessel element in which the scalariform perforations of the end wall are evident. In an earlier paper (Bliss, 1921), the writer figured portions of these vessels in section showing clearly the difference between the side and end walls (fig. 3). Typical vessels, in which the scalariform perforations of the end wall are also evident, are present in the root as well as in the stem of P. latiusculum (fig. 4, 5). In these figures the end wall is comparatively 1 Received for publication June 6, 1939. short, but in some cases the perforated area is very long. In one element, for instance (fig. 6), the writer counted 160 perforations; this indicates that the end wall must have met the side wall at a very small angle, and the area of contact must have been extensive. Through the courtesy of Professor Jeffrey of Harvard University the writer was able to investigate another species of Pteridium which he had collected in Australia-namely, P. esculentum (Forst.) Nakai. In this species, which is closely related to P. latiusculumn, vessels are also present in abundance. The elements vary in length, are clear cut in section, and are characterized in general by narrow scalariform bordered pits in the side wall and scalariform perforations in the end wall which is definitely at an angle with the side wall (fig. 7, 8). In a study of the material of both these species in root and stem, it is interesting to note that there are many cells that may be interpreted as transitional between the tracheid and the vessel element. In contrast to the typical vessel element where the end wall stands definitely at an angle with the side wall, there is often a cell that is a vessel element that tapers gradually to a point as does the tracheid and in which the area of contact between the contiguous elements in the terminal perforated region is much more extensive than it is in a typical vessel. This condition is evident in figure 9. There is every gradation from this very long end wall to the short end wall, oriented almost at right angles to the side wall and showing as few as six perforations. These conditions offer a possible explanation of the origin of the vessel from the tracheid in this genus. Following the investigation of Pteridiumn, nine species of Pteris were studied. In none of these was there evidence of vessels such as have been described for the two species of Pteridium. The elements were in all cases extremely long, of medium width, and narrowly tapering at the ends. These observations on the length and width of the tracheal elements in the Ferns confirm those made in this group by Bailey and Tupper (1918) in their study of the comparative length and width of the tracheal elements of the vascular cryptograms, Gymnosperms, and Angiosperms. Figure 10 illustrates the type of tracheal element characteristic of the genus Pteris L. In most cases the lateral wall is marked by a single series of elongated slit-like, scalariform pits, uniform throughout the length of the element. There are undoubted instances of serial and irregular pitting in the lateral walls of the element as noted by GwynneVaughan (1908), but in many cases, if traced far enough, the type of pitting is seen to be related to the fact that the element is overlain by parenchyma cells and is not here in contact with another tracheal element.