Outline of the classification of flowering plants (Magnoliophyta)

Trichomes occur in a multitude of forms and sizes. Although they have been used widely for taxonomic purposes, their adaptive significance has been all but ignored by the evolutionist and ecologist. It is clear that trichomes play a role in plant defense, especially with regard to phytophagous insects. In numerous species there is a negative correlation between trichome density and insect feeding and oviposition responses, and the nutrition of larvae. Specialized hooked trichomes may impale adults or larvae as well. Trichome may also complement the chemical defense of a plant by possessing glands which exude terpenes, phenolics, alkaloids or other substances which are olfactory or gustatory repellents. In essence, glandular trichomes afford an outer line of chemical defense by advertising the presence of \"noxious\" compounds. In some groups of plants, protection against large mammals is achieved by the presence of stinging trichomes. Intraspecific variation for trichome type and density is known in many species, and often is clinal in accordance with ecographic parameters. The presence of such correlations does not imply that differences in predator pressure are the causal factors, although this may indeed be the case.

role of trichomes in plant defense

Comparative Wood Anatomy

The richness of plant-life forms, ranging from unicellular algae to large trees, from trailing herbs to large climbing vines, and from ephemerals to long-lived perennials, suggests that a variety of environmental factors exert important influences on plant shapes (Terborgh 1973). In this paper we describe a model in which the inevitable association between water loss and entrance of carbon dioxide through stomates, together with cell-morphological and physiological traits that affect these exchanges, prevent any one plant-life form from being the best adapted to more than a fraction of the earth's complex patterns of temperature and moisture availability. These relationships may also explain apparently stable mixtures of plants with different life forms in a single environment. The tight linking of water loss and carbon uptake affects all aspects of the total photosynthetic and materials uptake system of vascular plants, because reducing water loss lowers rates of carbon uptake and hence lowers rates of net photosynthesis per unit area of photosynthesizing surface. The combination of characters that yields maximum net photosynthesis per unit time under moist conditions reduces the maximum rate under drier conditions, and vice versa. There are, of course, mechanisms, such as C4 photosynthesis and opening stomata only when atmospheric humidity is high, which reduce water loss per molecule of carbon fixed, but these mechanisms are of limited effectiveness and have associated costs. Any model of natural selection assumes both some "goal" that is being optimized (or maximized) and the constraints within which the organisms operate. A reasonable short-range goal for plants might be the maximization of photosynthetic rate; a plant capable of increasing photosynthesis within the constraints of its available resources and the physical environment should gain advantages in competition with other plants, defenses against herbivores, and should have more energy to devote to reproduction. We assume in our model that total fitness is strongly correlated with net gain in calories and that it is reasonable to treat allocations of resources to leaves, stems, and roots in terms of calories. Thus, we ignore other potentially significant factors, e.g., nonuniform mortality risks due to herbivory or pathogens, which should influence the

A cost-income model of leaves and roots with special reference to arid and semiarid areas

The eudicot order Malpighiales contains ∼16000 species and is the most poorly resolved large rosid clade. To clarify phylogenetic relationships in the order, we used maximum likelihood, Bayesian, and parsimony analyses of DNA sequence data from 13 gene regions, totaling 15604 bp, and representing all three genomic compartments (i.e., plastid: atpB, matK, ndhF, and rbcL; mitochondrial: ccmB, cob, matR, nad1B-C, nad6, and rps3; and nuclear: 18S rDNA, PHYC, and newly developed low-copy EMB2765). Our sampling of 190 taxa includes representatives from all families of Malpighiales. These data provide greatly increased support for the recent additions of Aneulophus, Bhesa, Centroplacus, Ploiarium, and Rafflesiaceae to Malpighiales; sister relations of Phyllanthaceae + Picrodendraceae, monophyly of Hypericaceae, and polyphyly of Clusiaceae. Oxalidales + Huaceae, followed by Celastrales are successive sisters to Malpighiales. Parasitic Rafflesiaceae, which produce the world's largest flowers, are confirmed as embedded within a paraphyletic Euphorbiaceae. Novel findings show a well-supported placement of Ctenolophonaceae with Erythroxylaceae + Rhizophoraceae, sister-group relationships of Bhesa + Centroplacus, and the exclusion of Medusandra from Malpighiales. New taxonomic circumscriptions include the addition of Bhesa to Centroplacaceae, Medusandra to Peridiscaceae (Saxifragales), Calophyllaceae applied to Clusiaceae subfamily Kielmeyeroideae, Peraceae applied to Euphorbiaceae subfamily Peroideae, and Huaceae included in Oxalidales.

Malpighiales Phylogenetics: Gaining Ground on One of the Most Recalcitrant Clades in the Angiosperm Tree of Life

Anatomy of the Dicotyledons

In certain woods, whose rays have extensive uniseriate margins, an individual marginal cell may sometimes be modified, by the perforation of its side walls, to connect two vessel segments on opposite sides of the ray. This feature does not appear to have been previously described though it is by no means uncommon; it has been observed by the authors in woods of many widely separated families. It is limited to woods with particular types of ray and vessel structure, but its occurrence seems to be primarily determined by the manner in which the vessels develop. Large ray cells which appeared to have scalariform perforations in the side walls were first observed by the authors in the wood of Lacistema aggregatum (Berg). Rusby. (Lacistemaceae); examination of macerated material, fig. 1, has shown that the side walls of these cells are exactly similar to the end walls of the vessel segments, and though it has not been possible to demonstrate the absence of a membrane, the authors are convinced from a careful study of the sections, that these walls are actually perforated, and that each forms a link between two vertical series of vessel segments. The course of the vessel is unusual; instead of continuing up and down between the rays, it periodically passes obliquely through a ray, and continues on the other side. The ray is usually uniseriate at the point at which it is crossed, and the ray cell involved becomes slightly swollen, rather in the manner of an oil or mucilage cell but without visible contents. The lateral walls are perforated, so that the cell resembles a very short vessel segment, and presumably functions as such. From the position and shape of these cells, however, it is clear that they have been derived from ray initials and not from fusiform initials. Attention was first drawn to this phenomenon by the presence of scalariform bars in the walls of cells which, from their shape, appeared at first sight to be oil or mucilage cells; simple perforations in such cells are obviously more easily overlooked, and it was not until a special search was made that they were found to be by no means uncommon, and to occur in woods of several different families.

M. M. Chattaway

Papers

Anatomy of the Dicotyledons

Perforated Ray Cells