Cornelius H. Muller

Dominance of California Annual Grasslands by Brassica nigra

Brassica nigra (L.) Koch. is a dicotyledonous member of the annual grassland vegetation of southern California. Pasture grassland areas often show pure stands of this species surrounded by areas inhabited by various mixtures of the grassland associates. Yearly regeneration of these pure stands occurs at the first rainfall of late autumn and initiates the growing season of all annual species of this vegetative type. At this time the weedy grass species which characterize these annual grasslands are notably absent in the areas of Brassica dominance. The exclusion of the grasses, such as Avena fatua, Bromus rigidus and Bromus mollis occurs primarily as a result of an allelopathic mechanism, involving the leaching of water soluble toxins from standing dead stalks and dead leaf material remaining in the soil. The pattern of geographic isolation of the species was shown not to be a result of spatial differences in soil texture, pH or temperature. Competitive mechanisms involving light or soil factors failed to be effective in this case of interference. Animal foraging was shown to be of minimal consequence. A potentially highly allelopathic, volatile toxin from living vegetative parts of Brassica was shown to lack an ecologically effective mechanism. Exudates from germinating seeds exhibited no effect upon grass seed in the laboratory and the possibility that this mechanism could be operative under field conditions is negligible. Water soluble phytotoxins, however, provide an ecologically effective mechanism which is operative in this field-inhibition phenomenon. Water soluble toxins inhibit the annual grassland species and permit the invasion and control of extensive grassland areas by Brassica nigra. INTRODUCTION Brassica nigra (L.) Koch. is a dicotyledonous member of the annual grassland vegetation of southern California. This vegetation is composed of a mixture of many species of grasses and broad-leaved herbs, though the grasses usually predominate (Heady, 1956; McNaughton, 1968). Perennial species vary in abundance depending on location, but generally comprise less than 1% of the plant cover (Talbot and Biswell, 1942). Like most of the dominant grasses, B. nigra is an introduced species, native to Europe and the Mediterranean. Some of the most common associates include Avena fatua, Bromus rigidus and B. mollis (McNaughton, 1968). All the aforementioned species can be characterized as weedy opportunists which quickly take advantage of any form of disturbance. This vegetational type has now generally re1 From a dissertation for the doctorate submitted by the first author to the Graduate Division of the University of California, Santa Barbara. 2 Financial support from National Science Foundation Grants GB4058 and GB6814 to C. H. Muller. 3 Present address: Department of Forestry, University of Illinois, Urbana 61801.

Allelopathic Mechanisms of Arctostaphylos glandulosa var. zacaensis

Arctostaphylos glandulosa var. zacaensis is a strong dominant of chaparral vegetation on Zaca Ridge in the San Rafael Mountains, California. It occurs in pure stands, sometimes openly spaced with the canopies covering 50 to 75% of the soil surface, or it may form completely closed cover. A barren soil surface, without seed- lings of any species in spite of sufficient light and soil moisture, char- acterizes the stands. Fire or artificial removal of shrubs results in growth of herbs and shrub seedlings until shrub dominance is again established. Detrimental biochemical interference (allelopathy) between Arcto- staphylos and associated plants is involved in this phenomenon. Aqueous leachate from Arctostaphylos foliar branches and from leaf litter is toxic to the growth of annual grasses. The allelopathic constituents leachable from Arctostaphylos plant organs have been identified as arbutin, hydro- quinone, and gallic, protocatechuic, tannic, vanillic, chlorogenic and p-hydroxybenzoic acids. Toxins such as p-hydroxybenzoic, p-coumaric, ferulic, syringic and o-coumaric acids are found in the soils beneath Arctostaphylos shrubs. These toxins can inhibit the germination and suppress the radicle growth of herbaceous plants at concentrations below 400 ppm; e.g., hydroquinone significantly inhibits radicle growth of Avena fatua and Bromus rigidus at a concentration of 50 ppm. Chem- ical constituents present in leaf litter appear to be more leachable after subjection to temperatures up to 160 C-resulting in increases of toxic- ity to herb growth-but are denatured at temperatures above 180 C, with complete loss of toxicity at temperatures above 200 C. Allelopathy

Plants as Indicators of Climate in Northeast Mexico

Thornthwaite (1931) has mapped North America climatologically from data collected in meteorological observatories. His method of computing climatic values for the area lying between two observatories is to average the data of the two known loci with no seeming regard for physiographic features lying between, which might (and in many cases do!) profoundly influence the climate of the intervening area. Such apparently was the reason why the climate of the Sierra Madre Oriental was designated by him to be the same as that of the surrounding plains and foothills from which were taken the only available data.

Mechanically Initiated Bark Growth in Celtis

While engaged during the spring of 1938 in making a vegetational survey of the Pere Marquette Wildlife Study Area for the Illinois State Natural History Survey, the author chanced to observe several examples of an odd growth phenomenon. The Pere Marquette Area is located on a point of land in Calhoun County, Illinois, at the juncture of the Illinois and Mississippi Rivers and consists entirely of river-bottom land which is for the most part wooded with typical river-bottom forest.

Further Studies in Southwestern Oaks

The descriptions of new species and other categories here recorded are made as one of a series of steps preliminary to an ultimate revision of the oaks of the southwestern United States and northern Mexico. In making such a study care is being taken to consider as significant only those physical geographic features which seem to have resulted in real evolutionary change. Political boundaries especially are disregarded.

Studies in Mexican and Central American Plants

The author encountered several trees of an apparently new Ulmus growing in a moist canyon in the mountains of northern Coahuila. The locality has an altitude of probably four or five thousand feet. The twig and foliage characters of the new material can hardly be used to distinguish it from Ulmus divaricata C. H. Mull.2 which occurs in similar habitats in central Nuevo Leon and has a similar habit. The trees observed in Coahuila were all barren of flowers and fruit at the time, but a single fallen fruit and some peduncles were found after a tedious search in debris-filled crevices. The differences exhibited by this single fruit (Fig. 1, B) are so profound that it is difficult to imagine that they might have arisen as the result of pathology, simple mutation, or ecological variation. The more numerous available fruits of U. divaricata exhibit a normal variation in form (e.g., about ten percent of the fruits have erect style branches rather than divergent ones), but in no case is the disposition of the principal veins and reticulum noticeably different from the illustration (Fig. 1, A) or in any way similar to the material here described as a new species.

The Legitimacy of Rejected Names Segregated from Conserved Genera

There have been a number of instances of conserved generic plant names and their corresponding rejected synonyms based upon specifically distinct types. In a few instances these type species have been segregated into separate genera by subsequent workers with narrower generic concepts. If the type of a rejected name is excluded from the genus bearing the corresponding conserved name, there arises the problem of whether the rejected name becomes legitimate or whether a new name must be supplied. In some cases new names have been proposed on the basis of the rejection of the only validly published name.

A New Species of Agave from Trans-pecos Texas

Folia anguste ovato lanceolata 45-55 x 6-7 cm.; spina 3-5 cm. longa attenuata lateribus applanta; margo ab apice ad basim ubique callosa; dentes 8-13 mm. longi reflexi. Scapus 2.5-3 m. longus basi 6-8 cm. crassus; inflorescentia 2.5-3 m. alta strictissima, ramulis 4-5 cm. longis, 10vel 12-floris, pedicellis 3-7 mm. longis. Flores 4.5 cm. longi, tubo 8 mm. longo, segmentis 22 x 7 mm., filamentis basi segmentorum insertis, 40-50 mm. longis stylum aequantibus. Fructus 35-40 x 17 mm.