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We investigated fruit and seed morphology in tribe Sorbarieae and one traditionally included genus (Lyonothamnus), which revealed that these characteristics are taxonomically significant for generic delimitation in the tribe Sorbarieae, and two distinct groups in the genus Sorbaria were identified. The following fruit and seed micromorphological characteristics were described: size, shape, texture, the presence/absence of epicuticular wax and, trichomes, testa cell size, and surface ornamentation. Multicellular capitate glands with stalks were only observed on the follicle surfaces of Lyonothamnus. Simple unbranched unicellular non-glandular trichomes were found on the follicles of Chamaebatiaria millefolium, Sorbaria grandiflora, S. sorbifolia complex, and Spiraeanthus schrenkianus. However, Sorbaria arborea complex, S. kirilowii, and S. tomentosa complex had platelet epicuticular wax on their glabrous follicles. Moreover, four types of testa ornamentation were identified: type I, colliculate (Lyonothamnus); type II, reticulate, with straight anticlinal walls and smooth to micro-papillate periclinal walls (Chamaebatiaria); type III, reticulate, with undulate anticlinal walls and smooth periclinal walls (Spiraeanthus); and type IV, reticulate testa ornamentation and divided into two subtypes based on anticlinal and periclinal cell wall patterns (Sorbaria). These studied characteristics identified using principal component analysis may have diagnostic importance among the taxa in the tribe. Our fruit and seed micromorphological results provide strong evidence that genus Lyonothamnus could be positioned in the tribe Lyonothamneae, not Sorbarieae. Moreover, these data supported the preliminary phylogenetic hypothesis that Sorbaria could be divided into two groups.

We conducted a comparative anatomical study of the petiole of 16 taxa belonging to the tribe Sorbarieae (Rosaceae) (Adenostoma, 2 spp.; Chamaebatiaria, 1 sp.; Sorbaria, 6 spp., 3 vars., and 1 forma; and Spiraeanthus, 1 sp.) and the related genus Lyonothamnus (1 sp. and 1 ssp.). The distal, medial and proximal regions of petioles were transversely sectioned using conventional embedding and staining methods. Cuticles, crystals, trichomes and pericyclic fiber patterns were observed and studied. Three types of vascular nodal patterns were recognized: Type 1 was seen in Chamaebatiaria, Lyonothamnus, and Spiraeanthus (simple‐trace nodal pattern with slightly curved or U‐shaped vascular bundle); type 2 was found in Adenostoma (multiple‐traces nodal pattern with free vascular bundles); and type 3 was unique to Sorbaria (bundles fused to form a siphonostele nodal pattern). Some petiolar anatomical characteristics (e.g. cuticles, crystals, trichomes, vascular nodal pattern, and pericyclic fiber patterns) were found to provide useful information for taxonomic studies within Sorbarieae. On the basis of these characteristics, a dichotomous key for identification at the generic/specific level is provided. We also report a structural change in the vascular bundles from the stem‐leaf transitional zone to the leaf medial zone.

Lyonothamnus Gray is a monotypic genus of evergreen trees endemic to California's Channel Islands. There is one species, L. floribundus, with two subspecies, ssp. floribundus and ssp. asplenifolius. Subspecies floribundus has simple and entire-margined leaves, while leaves of ssp. asplenifolius are pinnate and composed of three to seven primary segments. Despite its restricted modern occurrence, three fossil species have been described from Neogene paleofloras in the far western United States. Lyonothamnus mohavensis Axelrod is known from California; L. parvifolius (Axelrod) Wolfe and L. cedrusensis Axelrod, from Nevada. However, the size ranges used to segregate these species were all found to fall within the range of L. parvifolius from Stewart Valley, Nevada. Study of intact leaves reveals that the combination of (1) number of primary segments, (2) length of primary segments, (3) number of medial secondary segments per primary segment, and (4) lengths and widths of medial secondary segments is important in segregating species. Using these characters, L. parvifolius and L. wolfei Erwin et Schorn sp. nov. are recognized from Nevada and Oregon, and two species, L. mohavensis and L. axelrodii Erwin et Schorn sp. nov., from California. This renewed taxonomy within an updated temporal framework provides an important first step toward understanding the evolution, diversity, paleoecology, and paleobiogeographic history of this unusual member of the Rosaceae.

The presence of Lyonothamnus in the Furnace Creek formation (undescribed) is discussed from the standpoint of plant distribution in arid regions. It is suggested that the Catalina ironwood may have lived in canyons adjacent to the Furnace Creek basin during late Miocene or early Pliocene time, when rainfall in these areas is considered to have ranged from 13 to 15 inches annually.

1. In its floral morphology Lyonothamnus possesses both saxifragaceous and rosaceous features. Development of the carpels before all the stamens are formed agrees with some of the Rosaceae and a few of the Saxifragaceae. The one-celled archesporium, the failure to form a "coiffe epidermique," the absence of an obturator, and the development of a cellular basal apparatus are features purely saxifragaceous in nature. The differentiation of many nucellar cells which exhibit characters typical of spore mother cells indicates a close relationship with the Rosaceae. The peculiar elongation of the megagametophyte prior to fertilization, and its seed characters (simple modification of the integument, large embryo with a pair of fleshy planoconvex straight cotyledons, unenlarged basal suspensor cell, and sparingly developed endosperm) are features found among the Rosaceae. 2. While many of the floral characters of Lyonothamnus are either saxifragaceous or rosaceous in nature, the formation of two series of ovules ...