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Absorption from culture solution of the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 3-(p-chlorophenyl)-1, 1-dimethylurea (Monuron) by excised barley (Hordeum vulgare L.) roots was studied to determine whether absorption was due to an active or a passive mechanism. Herbicide absorption was followed at low temperature, under anaerobic conditions, and in the presence of metabolic inhibitors and compounds of structure similar to that of the herbicide. Total absorption was divided into two phases, exchangeable and nonexchangeable herbicide, by washing the roots for 1 hour following absorption. Absorption of both exchangeable and nonexchangeable 2,4-D appeared to depend on a supply of metabolic energy which suggests that an active mechanism may be involved. A possible conclusion is that 2,4-D is absorbed by roots by an adsorption mechanism and that energy is required to maintain the integrity of the absorbing surfaces of the cell. In contrast, absorption of Monuron was independent of an energy supply. It is concluded that the bulk of the Monuron absorbed was taken up passively by diffusion.Absorption of mineral salts by plants has been extensively studied and much information on this subject now exists. Although many organic compounds, such as insecticides, fungicides, and herbicides, are applied either directly to plants, or to the soil from where they are taken up by the roots, less information is available on the absorption of organic substances.Both physical and metabolically controlled processes have been shown to control the absorption of indoleacetic acid by pea epicotyl segments and carrot disks (20), and Avena coleoptile sections (17). With Lemna minor the initial uptake of 2,2-dichloropropionic acid (Dalapon) was concluded to be largely under the control of physical factors, whereas the subsequent accumulation was metabolically controlled (18). Absorption of mercuric acetate and phenylmercuric acetate by excised and intact pea roots shows both a passive and an active phase of uptake (19). Hancock (10), using diseased and healthy squash hypocotyls, found that 3-o-methylglucose is taken up in

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In the preceding paper (7) it was noted that the true mistletoe, Phoradendron, derives very little if any carbohydrate from its host, while the dwarf mistletoe, Arceuithobiutm, draws heavily upon the photosynthate of its host. These findings imply that Phoradendron is capable of furnishing its own energy requirements but say little regarding the extent of dependence of Arceuthobium upon its host for carbohydrates.The aerial shoots of Arceuthobium frequently exhibit an off-green color which Peirce (9) claimed is due to an abundant supply of chlorophyll that is often masked by dark pigments in the epidermal cells. Heinricher (6) described green-colored bodies in the cells of the endophytic system, however, this observation was not confirmed by Peirce (9)

BRUSHY SPECIES of plants occupy millions of acres of land in California. Some of this land is rough, rocky, and unsuited for the growth of grass. Much of it, however, is well suited to the growth of forage species, and only the cover of brush prevents its use for grazing. Vigorous efforts are being made to reclaim this valuable land (Love and Jones, 1952) by controlled burning, reseeding, and improved range-management practices. The weak link in this program is the control of seedlings and resprouting stumps of certain brush species. Although reburning and proper 'grazing management are useful, chemical control is proving most valuable in this final clean-up process. For successful chemical control of brushy plants, the chemical must penetrate the leaves or bark and move through the stem into the roots. Most resprouting occurs from a band of meristematic tissue at or somewhere below the ground line. In young seedlings this tissue is rather easily destroyed; in old plants it becomes more difficult to kill. Although 2,4-dichloro-and 2,4,5-trichlorophenoxyacetic and propionic acids are now in use, further studies of these four basic molecules must be carried out in many localities to determine the effects of differences in species, habitats, seasonal responses, and methods of application. Obviously, a radioactive tracer that can be followed after it is applied to leaves or bark can be a valuable tool in studies on absorption and translocation. This report deals primarily with the absorption, translocation, and toxic action of 2,4-D. The carboxyl group of the 2,4-D used in these experiments was labeled with C14. This tracer, hereafter referred to as 2,4-D*,' was applied to several California species of brushy plants. In conjunction with the tracer studies, branch tips were treated with nonradioactive 2,4-D to determine the extent of die-back in the various species. Two related studies were also undertaken: one a comparative analysis of the penetrative capacities of two formulations of 2,4-D*, the other a comparison between radioactive urea (urea*) and 2,4-D*. STUDIES IN ABSORPTION AND TRANSLOCATION Two areas representing different soil and climatic conditions were selectedone west of Davis in the foothills of the Coast Range, the other to the east in the Sierra. The growing season in the west area is early; that in the east is about a month later. The west area has rather low rainfall, the east has more. By making periodic treatments and samplings in both areas, it was possible to cover many growth stages, several species, several sites, and rather distinct differences in humidity and temperature. Seven species were studied: blue oak, Quercus douglasii; wedge-leaf ceano-1 Received for publication June 10, 1955. 2 Associate Botanist in the Experiment Station, Botany Department) Davis. :1 Professor of Botany and Botanist in the Experiment Station, Davis. 4 Supplied by Tracerlab.