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MacKinney, G. 1941. Absorption of light by chlorophyllll solutions. J. Biol. Chem. 140:315-322. Mayr, H. 1909. Waldbau auf naturgesetzlicher Grunidlage. Paul Parey, Berlin. 568 p. McGregor, W. H. D., R. M. Allen, and P. J. Kramer. 1961. The effects of photoperiod on growth, photosy nthesis, and respiration of loblolly pine seedlings from two geographic sources. Forest Sci. 7:342-348. Mergen, F. 1963. Ecotypic variation in Pinuis strobuts L. Ecology 44:716-727. Milner, H. W., W. M. Hiesey, and M. A. Nobs. 1959. Physiology of climatic races. Carnegie Institution of Washington Year Book 58:346-450. . 1960. Physiology of climatic races. Carnegie Institution of Washington Year Book 59:313-318. Mooney, H. A., and W. D. Billings. 1961. Comparative physiological ecology of arctic and alpine populationis of Oxyria digyna. Ecol. Monographs 31:1-29. Scholander, Susan I., and Joan T. Kanwisher. 1959. Latitudinal effect oni respiration in some northern plants. Plant Physiol. 34:574-576.

1. Aqueous extracts of Ailanthus leaves collected during June, July, and October were applied to slash pine and Monterey pine seed and seedlings. A decrease in toxicity was observed in the October collection. 2. Sterilization of the soil had no effect on the survival of transplanted seedlings, but when seeds were planted in sterilized soil and watered with Ailanthus extract, a detrimental effect was observed. 3. The addition of Ailanthus leaves to the soil did not inhibit the activity of soil organisms as measured by the production of CO2.

1. The cytology of the various stages during fertilization in Austrian pine (Pinus nigra Arnold) are described. 2. A photographic record is presented of the sequence from the time when the two sperm nuclei enter the egg until the first two free nuclei of the zygote are formed.

1. The presence of a substance that was toxic to other tree seedlings was demonstrated in aqueous extracts of foliage from Ailanthus altissima (Mills) Swingle. 2. The effect of concentration of, and the response to, this extract was tested on thirty-five species of gymnosperms and on eleven species of angiosperms. Members of all species, with the exception of those of Fraxinus americana L., were adversely affected.

A Cesium 137 (9500 c) source was placed in a natural pitch pine—oak forest at the Brookhaven National Laboratory. Beginning on November 22, 1961, the forest was exposed to gamma rays during 20 hr each day. Pinus rigida trees were exposed to radiation levels ranging from a high of 960 r/day to a level of normal background radiation. Microsporangiate strobili were collected and evaluated at regular intervals to follow the effects of gamma radiation on the cytology of microsporogenesis. For comparison, mature pollens of Pinus rigida and Pinus strobus were exposed to acute γ radiation for a period of 16 hr from a Cobalt 60 source of approximately 3200 c at levels ranging from 1 to 300 kr. Trees receiving 82 r/day and higher from Cs 137 were unable to resume cell division and differentiation during the spring of 1962, while microspore mother cells were formed on trees receiving 75 r/day. At 56 r/day meiosis with susbequent formation of tetrads took place, and the highest level at which viable microspores were collected was 11 r/day. A slight reduction in the length of the strobili was observed at 0·4 r/day, and at 3·7 r/day they were significantly shorter than the controls. The percentage of cells with visible chromosome aberrations increased with an increase in exposure, e.g. 0·4 r/day=1 per cent; 7 r/day=2 per cent; 11 r/day=12 per cent; and 17 r/day=76 per cent. Pollen abortion increased with an increase in radiation, and at 7 r/day, approximately 50 per cent of the pollen grains were aborted. At 3 to 5 r/day, germination in vitro was reduced to 50 per cent of the controls. Pollen tube length was not related to exposure. Mature pollen of Pinus rigida and Pinus strobus produced normal pollen tubes in vitro after 16 hr exposure to 300 kr Co 60 radiation. Illustrations of the various observations are given, and the results are discussed in relation to physiological and genetic implications.