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Abstract. Electron micrographs are presented which establish the identity of the components of the 3 major bands observed after sucrose density centrifugation of the crude particu,late fraction from the endosperm of germinating oastor bean seedlings. These are: mitochondria (density 1.19 g/cc), proplastids (density 1.23 g/cc) and glyoxysomes (density 1.25 g/cc). Further evidence is provided on the enzymatic com.position of the glyoxysomes. Essentially all of the partioulate malate synthetase, isocitrate lyase, catalase, and glycolic oxidase is present in these organelles. The distribution of glyoxysomal enzymes on sucrose density gradients is contrasted with that of the strictly mitoclhondrial enzymes fumarase, NADH oxidase, and succinoxidase. Malate dehydrogenase and citrate synthetase are present in both organelles. The functional role of glyoxysomes and their relationship to cytosomes from other tissues is discussed.A previotus report (5) established that when crude pairticullate preparations from germinating castor bean endospernm were centrifuged on stucrose density gradients, 3 major bands were obtained. The enzymes of the glyoxylate cycile were confined to the densest o(f these ban'ds, and the particles c'ontained in that band were therefore named glyoxysomes.In this report electron micrographis are presented which show the morphology and degree oif homogeneity of the particles in each o'f the bands, respectively mitochondria, propla,stids and glyoxysomes. M'ore detailed information i,s provided to show the ditstributiion of mito-chondrial and glyoxylate cycle enzymes in sucrose density gradients, and evidence for the association of catalase and glycolic oxidase with the glyoxysomes is presented. Materials and MethodsCastor bean 'seed's were grown and their endosperms were harvested and homogenized a's previously described (5). Pellets containing a mixture of celil ,particles were resuspenided and placed on stucrose den'sity gra'dients fabricated as described in reference 5 (noted in this paper as gradient system I) or on gradients consiisFting of 25 ml of sucrose g,rading from 60 % to 30 % (gradient system II) and contained. in cellu-lolse n,iitrate tubes for the Spinco SW25.2 rotor. All gradients contained 1 Reseanch supported in part by NSF Grants GB 2599 and GB 2897 and AEC Contract 11-1-330. EDTA at 10-3 M. Cent-rifulgatio1l was carried ott for 4 hours ait 25,000 rpm and 00 anid 1 ml fractions were colllectedl as described (5)

Abstract.A thermodynamic model describing the relation between plant growth and respiration rates is derived from mass-and enthalpy-balance equations. The specific growth rate and the substrate carbon conversion efficiency are described as functions of the metabolic heat rate, the rate of CO2 production, the mean oxidation state of the substrate carbon produced by photosynthesis, and enthalpy changes for conversion of photosynthate to biomass and CO2. The relation of this new model to previous models based only on mass-balance equations is explored. Metabolic heat rate is shown to be a useful additional measure of respiration rates in plant tissues because it leads to a more explicit description of energy relations. Preliminary data on three Zea mays (L.) cultivars are reported. The model suggests new rationales for plant selection, breeding and genetic engineering that could lead to development of plants with more desirable growth rates.