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2,4-Dichlorophenoxyacetic acid (2,4-D) promotes the accumulation of tryptophan-derived indole-3-acetic acid (IAA) (12,21,22). This endoge-' These studies were supported by U.S. Department of Agriculture competitive research grant 89-37261-4791

Despite the vast diversity of sizes and shapes of living organisms, life's organization across scales exhibits remarkable commonalities, most notably through the approximate validity of Kleiber's law, the power law scaling of metabolic rates with the mass of an organism. Here, we present a derivation of Kleiber's law that is independent of the specificity of the myriads of organism species. Specifically, we account for the distinct geometries of trees and mammals as well as deviations from the pure power law behavior of Kleiber's law, and predict the possibility of life forms with geometries intermediate between trees and mammals. We also make several predictions in excellent accord with empirical data. Our theory relates the separate evolutionary histories of plants and animals through the fundamental physics underlying their distinct overall forms and physiologies.allometric scaling | biological scaling | tree geometry | fractal U nderstanding the origin and evolution of the geometries of living forms is a formidable challenge (1, 2). The geometry of an object can be characterized by its surface−volume relationship-the surface area S of an object of volume V can scale at most as S ∼ V and at least as S ∼ V 2=3 (3). These geometries have been used by nature in space-filling trees and animals, respectively. Here, our principal goal is to explore how it is that both geometries of life coexist on Earth, whether intermediate geometries are possible, and what all this implies for evolution of life on Earth.Living organisms span an impressive range of body mass, shapes, and scales. They are inherently complex, they have been shaped by history through evolution and natural section, and they continually extract, transform, and use energy from their environment. The most prevalent large multicellular organisms on Earth, namely plants and animals, exhibit distinct shapes, as determined by the distribution of mass over the volume. Animals are able to move and are approximately homogeneous in their mass distribution-yet they have beautiful fractal transportation networks. Plants are rooted organisms with a heterogeneous selfsimilar (fractal) geometry-the mass of the tree is more concentrated in the stem and branches than in the leaves.The approximate power law dependence of the metabolic rate, the rate at which an organism burns energy, on organism mass has been carefully studied for nearly two centuries and is known as allometric scaling (4-32). From the power law behavior, with an exponent around 3/4, one can deduce the scaling of characteristic quantities with mass and, through dimensional analysis, obtain wide-ranging predictions often in accord with empirical data. However, what underlies this ubiquitous quarter-power scaling, and with a dominant exponent of 3/4?In an influential series of papers, West and coworkers (11, 12, 14-16) suggested that fractality was at the heart of allometric scaling. Inspired by these papers, a contrasting view was presented (13), which argued that, although fractal circulatory networks m...

Abstract. Plasmodesmata or intercellular bridges that connect plant ceils are cylindrical channels •40 nm in diameter. Running through the center of each is a dense rod, the desmotubule, that is connected to the endoplasmic reticulum of adjacent ceils. Fern, Onoclea sensibilis, gametophytes were cut in half and the cut surfaces exposed to the detergent, Triton X 100, then fixed. Although the plasma membrane limiting the plasmodesma is solubilized partially or completely, the desmotubule remains intact. Alternatively, if the cut surface is exposed to papain, then fixed, the desmotubule disappears, but the plasma membrane limiting the plasmodesmata remains intact albeit swollen and irregular in profile. Gametophytes were plasmolyzed, and then fixed. As the cells retract from their cell walls they leave behind the plasmodesmata still inserted in the cell wall. They can break cleanly when the cell proper retracts or can pull away portions of the plasma membrane of the cell with them. Where the desmotubule remains intact, the plasmodesma retains its shape. These images and the results with detergents and proteases indicate that the desmotubule provides a cytoskeletal element for each plasmodesma, an element that not only stabilizes the whole structure, but also limits its size and porosity. It is likely to be composed in large part of protein. Suggestions are made as to why this structure has been selected for in evolution.

Abstract. In response to increasing calls for the reform of the undergraduate science curriculum for life science majors and pre-medical students (Bio2010, Scientific Foundations for Future Physicians, Vision & Change), an interdisciplinary team has created NEXUS/Physics: a repurposing of an introductory physics curriculum for the life sciences. The curriculum interacts strongly and supportively with introductory biology and chemistry courses taken by life sciences students, with the goal of helping students build general, multi-discipline scientific competencies. In order to do this, our two-semester NEXUS/Physics course sequence is positioned as a second year course so students will have had some exposure to basic concepts in biology and chemistry. NEXUS/Physics stresses interdisciplinary examples and the content differs markedly from traditional introductory physics to facilitate this. It extends the discussion of energy to include interatomic potentials and chemical reactions, the discussion of thermodynamics to include enthalpy and Gibbs free energy, and includes a serious discussion of random vs. coherent motion including diffusion. The development of instructional materials is coordinated with careful education research. Both the new content and the results of the research are described in a series of papers for which this paper serves as an overview and context.

(D.M.R., N.I., T.J.C.)l h e effect of auxin application on auxin metabolism was investigated in excised hypocotyl cultures of carrot (Daucus carota). Concentrations of both free and conjugated indole-3-acetic acid (IAA), [2H,]IAA, 2,4-dichlorophenoxyacetic acid, and naphthaleneacetic acid (NAA) were measured by mass spectroscopy using stable-isotope-labeled interna1 standards.[l3C1]NAA was synthesized for this purpose, thus extending the range of auxins that can be assayed by stable-isotope techniques. 2,4-Dichlorophenoxyacetic acid promoted callus proliferation of the excised hypocotyls, accumulated as the free form in large quantities, and had minor effects on endogenous IAA concentrations. NAA promoted callus proliferation and the resulting callus became organogenic, producing both roots and shoots. NAA was found mostly in the conjugated form and had minor effects on endogenous IAA concentrations. [ZH,lIAA had no visible effect on the growth pattern of cultured hypocotyls, possibly because it was rapidly metabolized to form inactive conjugates or possibly because it mediated a decrease in endogenous IAA concentrations by an apparent feedback mechanism. l h e presente of exogenous auxins did not affect tryptophan labeling of either the endogenous tryptophan or IAA pools. This suggested that exogenous auxins did not alter the IAA biosynthetic pathway, but that synthetic auxins did appear to be necessary to induce callus proliferation, which was essential for excised hypocotyls to gain the competence to form somatic embryos.Since the 1940s (Thimann, 1974), exogenous auxins have been used frequently as experimental tools for studying their effects in developing structures (Schiavone and Cooke, 1987); for screening for mutants impaired in the uptake, metabolism, and physiology of endogenous auxins (Timpte et al., 1994); and even to find potential IAA receptors (Jones and Venis, 1989; LoSchiavo et al., 1991). One important use for the application of exogenous auxin is as a growth regulator for maintaining plant-cell-and tissue-

This article considers a multiyear conversation between a physicist interested in adapting a physics course for biologists and a biologist interested in including more physics in a biology course. Examples are given, along with insights developed about the different approaches biologists and physicists tend to take toward the same phenomena.