ONTOGENY OF THE OPEN DICHOTOMOUS VENATION IN THE PINNA OF THE FERN NEPHROLEPIS

PRAY, THOMAS R. (U. South. California, Los Angeles.) Ontogeny of the open dichotomous venation in the pinna of the fern Nephrolepis. Amer. Jour. Bot. 47(5): 319-328. Illus. 1960.-The venation of the pinna of Nephrolepis consists of a midvein and 2 lateral series of dichotomizing veins all of which terminate freely near the margins. The development of the pinna is analyzed with particular attention to the nature of the marginal meristem and the organization of the embryonic pinna as it appears in paradermal section. The arrangement of cells in pinna wings during the period of marginal growth displays a pattern which foreshadows the pattern of the mature venation. In contrast with the development of the leaves of angiosperms, marginal growth continues into a relatively late phase of pinna ontogeny and apparently is active throughout the phases of ontogeny concerned with blocking-out the pathways of procambial differentiation. Thus the pattern of venation appears to be correlated with the manner of activity of the marginal meristem and subsequent orientation of its derivatives. The theoretical aspects of the result of this investigation are discussed in relation to other studies of foliar venation ontogeny. THIS paper is concerned with the ontogenetic processes which result in the open dichotomous venation pattern of the pinnae of Nephrolepis exaltata. The prevalent pattern of dichotomous venation in the leaves of ferns offers an intriguing subject for investigation for several reasons. In its simplest form, a strictly open system of diverging veins, the pattern is in marked contrast to the various types of closed reticula which are nearly universal in angiosperm leaves. Also, within the ferns as a group, dichotomous venation has been considered primitive and various patterns of a reticulate nature as derived (Bower, 1923). It has been suggested recently, however, that fern leaves which display a dichotomous organization are derived from pinnate forms, at least so far as extant species are concerned (Wagner, 1952). In spite of the interest in fern foliar venation, the literature on leaf development in this group of plants is rather limited. The only information on venation ontogeny at the histogenetic level is contained in a sketchy account by Goebel (1922) of pinna development in a species of Adiantum. MATERIALS AND METHODS.-The leaf of Nephrolepis exaltata was selected for investigation because the young pinnae of the developing leaf of this commonly cultivated species are nearly flat during the earlier phases of ontogeny. Thus it was possible to obtain well-oriented paradermal sections of the pinnae which are necessary for a detailed study at the cellular level. Growth of the circinnate leaf tip is prolonged, frequently producing 50 or more lateral pairs of pinnae. Sectioning of a number of such tips at 8 u yielded good examples of the critical phases of pinna ontogeny. Some transverse sections of developing pinnae also were made. 1 Received for publication November 22, 1959. Cleared and stained specimens of mature and developing pinnae were prepared, but the latter proved unsuitable as a source of the precise information desired. The materials were fixed in formalinpropionic acid-ethyl alcohol and dehydrated and embedded by the tertiary butyl alcohol method (Johansen, 1940). The clearing procedures and the staining methods used for both cleared and sectioned materials were the same as those previously employed by the writer (Pray, 1955a). I am indebted to Dr. Sherwin Carlquist for his expert assistance in the preparation of the photographs used in this paper. RESULTS.-Venation pattern. The pinnae of the Nephrolepis form used in this study are 21/2-3 in. long with serrate margins and a basal lobe. The venation of a typical sterile pinnae (fig. 1) consists of a sympodial system with midvein and a series of alternating dichotomous lateral veins. In the distal portion, the laterals fork once; in the median and basal portions, the proximal or both branches dichotomize once more. The 2 basal laterals are somewhat more elaborate, particularly the one in the basal lobe. A secondary sympodial system of this latter is comparable to that of the pinna as a whole. Without exception, all branches of the venation system terminate freely and submarginally. All vein tips are markedly dilated into a hydathodic mass of short, broad tracheary elements. The venation of fertile pinnae is essentially the same with the sori located at the shortened tips of the distal member of each lateral branch. Pinna initiation.-The young leaf tip is occupied by a single apical initial with extended activity. This is true of all members of the Polypodiaceae (sensu lato) thus far investigated (Bower, 1923). The young rachis distal to emergent pinnae (fig. 2) is nearly cylindrical and continues to grow by [The Journal for April (47: 239-318) was issued April 20, 19601 AMERICAN JOURNAL OF BOTANY, Vol. 47, No. 5, May, 1960