The classification of the dwarf mistletoes comprising the Arceuthobium campylopodum-occidentale complex continues to be one of the most difficult taxonomic problems associated with this important group of parasitic flowering plants. The complex consists of four taxa: A. campylopodum Engelmann, A. occidentale Engelmann, A. littorum Hawksworth, Wiens & Nickrent, and A. siskiyouense Hawksworth, Wiens & Nickrent; all of which are morphologically similar, flower and disperse seed in the fall, and parasitize hard pines. We collected morphological measurements for these four taxa from throughout their geographic ranges and used non-parametric and multivariate statistical analyses to compare morphological differences among them. Our analyses demonstrated that male plants of A. littorum and A. siskiyouense can be delimited morphologically; therefore, we recommend that both taxa continue to be recognized as species. Although, our results indicated that A. occidentale is very similar morphologically to A. campylopodum, it can be distinguished from the latter species using morphological characters, particularly for female plants. Furthermore, because A. occidentale flowers and disperses seed nearly one month later than A. campylopodum, we recommend it continue to be treated at the species level pending additional study. Comparative morphological and phenological differences between and among the species studied herein are summarized and can be used for field/laboratory diagnosis of these taxa.

Serotiny and delayed seed dispersal are thought to have evolved as traits allowing plants to cope with environmental variability. We conducted a literature search of serotinous desert plants in the Mojave and Sonoran deserts in California and classified them according to their seed retention syndrome. Serotinous species in North American deserts have evolved similar seed retention syndromes as those of plants in other deserts of the world. Seed retention in deserts plants is a common seed dispersal strategy in arid ecosystems that allows plants to cope with environmental variability and unpredictability.

Se cree que la serotinia y la dispersión retrasada de semillas han evolucionado como un mecanismo que permite a las plantas hacer frente a la variabilidad ambiental. En el presente estudio realizamos una búsqueda en la literatura sobre las plantas serótinas de zonas áridas en los desiertos del Mojave y Sonorense en California. Las plantas encontradas fueron clasificadas de acuerdo al síndrome de retención de semillas. Las especies serótinas de los desiertos de Norte América han evolucionado síndromes de retención de semillas similares a los de las plantas de otros desiertos del mundo. La retención de semillas en plantas de desierto es una estrategia de dispersión común en los ecosistemas áridos que permite a las plantas adaptarse a la variabilidad e impredecibilidad ambiental.

Many fewer species of cacti are native to and thought to be able to survive winters in eastern Ontario than in the similarly cold winters of central Colorado. We collected 12 yr of data on 107 specimens representing 50 cactus species that have been tested outdoors in gardens in the City of Ottawa (Canada) and report which have successfully weathered six or more consecutive winters and which have not. Some species that we expected to be more successful were not, while a small number of species native to considerably warmer environments survived surprisingly well. We review general mechanisms for freeze-tolerance in plants, focusing on what is known about cacti in particular. Phylogeny does not appear to be important in determining success or failure in cold climates, so we explore other possible explanatory factors for differences in survival between Ontario and Colorado. Our data indicate that freeze tolerance of cacti in eastern Ontario may be a function of snow cover, rather than polyploidy. Colorado's greater cactus richness may also be a function of its location closer to the southwestern deserts' center of diversity, which would provide a larger pool of potential species that could expand to colder regions. More thorough studies of freeze-tolerance over a larger geographic range – albeit controlling for growing conditions, using identical clones at multiple sites, and determining precise cause of death – will be necessary to reach more definitive conclusions.

As a result of the author's doctoral biosystematic study of the genus Cylindropuntia in Baja California, plus subsequent field exploration and continued taxonomic research on the flora of this region in northwestern México, various new cactus taxa have been discovered. Seven new cacti including six chollas (Cylindropuntia alcahes (F. A. C. Weber) F. M. Knuth var. gigantensis Rebman, C. alcahes var. mcgillii Rebman, C. cedrosensis Rebman, C. ganderi (C. B. Wolf) Rebman & Pinkava var. catavinensis Rebman, C. libertadensis Rebman, and C. waltoniorum Rebman) and one new species of prickly-pear (Opuntia clarkiorum Rebman) endemic to the Baja California peninsula and adjacent islands are described here for the first time. Distribution, associated vegetation, rarity, affinities to other related species, botanical illustrations, and various keys for identification of these cacti in the region are also presented.

Como resultado del estudio biosistemático doctoral del autor del género de Cylindropuntia en Baja California, más exploración de campo y la posterior investigación taxonómica continúa en la flora en el noroeste de México, se han descubierto varios taxones de cactus endémica de esta región. Siete nuevos cactáceas incluyiendo seis chollas (Cylindropuntia alcahes (F. A. C. Weber) F. M. Knuth var. gigantensis Rebman, C. alcahes var. mcgillii Rebman, C. cedrosensis Rebman, C. ganderi (C. B. Wolf) Rebman & Pinkava var. catavinensis Rebman, C. libertadensis Rebman, y C. waltoniorum Rebman) y una nueva especie de nopal (Opuntia clarkiorum Rebman) endémica de la península de Baja California y las islas adyacentes se describen aquí por primera vez. Se presentan la distribución, vegetación asociada, rareza, afinidad con otras especies en el género, ilustraciónes botánicas del nuevos taxónes, y varias claves por la identificación de estos cactáceas en la región.

Imbribryum torenii J.R. Spence & Shevock is described and illustrated. This species appears related to I. alpinum (Hudson ex Withering) N. Pedersen and I. muehlenbeckii (Bruch & Schimper) N. Pedersen. Imbribryum torenii is readily distinguished from these two species by a combination of characters including broadly ovate leaves, short excurrent costa, hexagonal distal laminal cells, cylindric capsule, and large spores.