A new type of branch has been studied in three species of the genus Aphanolejeunea (Hepaticae). The Aphanolejeunea-type branch is similar to the Lejeunea-type branch of the closely allied genus Cololejeunea. The branch initial in the Aphanolejeunea-type branch is a superficial cortical cell, while the branch initial in the Lejeunea-type branch is a cortical cell-derivative enclosed by leaf brace cells. Leitgeb (1875) was the first to provide detailed descriptions of the variety of developmental sequences leading to the formation of lateral branches in the Hepaticae. Hepaticologists such as Evans (1912), Schuster (1966) and Crandall (1969) have all continued along this avenue of research, tracing the development of undescribed branch types, as well as surveying branch development in a wide range of species. One reason for this interest in branch development is that there seems to be some systematic significance attributable to the variety of different types found in the Jungermanniales. For example, primitive groups may produce a wide variety of branch types, as in the genus Temnoma Mitt., in which branches of four different types are found (Schuster 1966), but in a specialized genus such as Leptolejeunea (Spruce) Schiffn., only a single type of branch (the Lejeunea-type) is produced. Schuster (1966) related the plasticity in the branching mode of a taxon to the growth habit of the plant. He hypothesized that a primitive, erect, isophyllous plant could conceivably produce branches from any stem sector, with little selection pressure upon it to standardize the type of branch produced. However, with the assumption of a prostrate growth habit, and the development of anisophylly, he stated, . .. there has been a sharper and sharper restriction of branching modes to the point where some groups develop branches of only one single type. Even in those groups where branching has become highly restricted, the branch types formed may provide useful clues to phylogenetic relationships within the group. In the highly specialized family Lejeuneaceae, two types of branches are reported to subtend vegetative lateral leaves. These are the Frullania-type branch (Evans 1912), and the Lejeunea-type branch (Schuster 1963). The Frullania-type, which replaces a portion of a lateral leaf, is considered to be more primitive than the Lejeunea-type, which subtends a lateral leaf (Schuster 1966). Frullania-type branches are found only in the primitive subfamilies Nipponolejeuneoideae and Ptychanthoideae. Within the Ptychanthoideae, branch types continue to provide a useful basis for evolutionary comparisons of taxa, because, according to Crandall (1969), some species, such as Dicranolejeunea axillaris (Nees & Mont.) Schiffn., produce lateral branches exclusively of the Frullania-type; others bear both Frullaniaand Lejeunea-type branches, e.g. Stictolejeunea squamata (Willd.) Schiffn., and many species have only Lejeunea-type branches. In the more specialized subfamily Cololejeuneoideae, lateral branch type would not appear from the lit007-2745/82/104-109$0.75/0 This content downloaded from 157.55.39.17 on Wed, 06 Jul 2016 05:23:56 UTC All use subject to http://about.jstor.org/terms 1982] THIERS: BRANCHING IN LEJEUNEACEAE 105 erature to be a phylogenetically important character, as only Lejeunea-type lateral branches have been described for species of this subfamily. However, a previously undescribed branch type has recently been observed in three species of Cololejeuneoideae: Aphanolejeunea minuta Schust., Aphanolejeunea microscopica (Tayl.) Evans, and Aphanolejeunea contractiloba (Evans) Schust. The Aphanolejeunea-type branch is so named because it was observed only in that genus. It is a non-innovational, athecal branch that forms an approximate 900 angle with the stem. The branch does not replace any part of the lateral leaf it subtends; it neither modifies the position of the leaf relative to the stem, nor is it adnate to the leaf. The Aphanolejeunea-type branch is the sole type of non-innovational branch found in these three species, and it may subtend elobulate leaves as well as those with well-developed lobules. MATERIALS AND METHODS The following collections were used in this study: Aphanolejeunea minuta Schust. U.S.A., Florida, Highlands Co., Highlands State Park, Schuster 80-124 (herb. R. M. Schuster). Aphanolejeunea microscopica (Tayl.) Evans. Denmark, Faeroe Is., Stromo, Vesthavn, Jensen, 1898 (NY). Aphanolejeunea contractiloba (Evans) Schust. U.S.A., Florida, Citrus Co., Pineola, Fern Grotto, Schuster 80-284 (herb. R. M. Schuster). Cololejeunea minutissima (Sm.) Schiffn. U.S.A., Florida, Columbia Co., Hildreth, Ichetucknee Springs, Schuster 80-14 (herb. R. M. Schuster). Portions of these dried collections were rehydrated and some whole plants were mounted in Hoyer's Solution for examination. Other plants from some of the collections were prepared for paraffin embedding according to the method described for dried material by Crandall (1969). Sections were cut with a steel knife at a thickness of 10 gm with a Spencer rotary microtome. Drawings were made with the aid of a Bausch & Lomb camera lucida attached to an American Optical Microstar compound microscope. In addition, whole plants (mounted in H20) were photographed through a Reichert Zetopan microscope equipped with Nomarski optics to illustrate selected stages of Aphanolejeunea-type branch development.