- Gas exchange, leaf area development, leaf anatomy and stomatal characteristics of four tall fescue selections differing in forage yield were examined to provide further insight into characteristics associated with yield differences of these selections. Winter-growing TFM 26 and TFM 16, the highest and lowest-yielding selections, respectively, and summer-growing selections TFK 4 and TFK 12, high and low yielding, respectively, were studied. Carbon dioxide-exchange rate (CER) of ten genotypes per selection was measured in December, 1978, and April, 1979, using infrared gas analysis. In April, 1979, dark respiration (Rd) and transpiration (Tn) rates were also evaluated and resistances to gas diffusion were calculated. Specific leaf weight (SLW) of leaves used for gas exchange measurements was determined. Leaf area development, as described by leaf elongation (Le) rate, leaf width (w) and leaf area expansion (La) rate, was assessed during three measurement periods. Photomicrography of leaf cross-sections and microscopic observation of leaf impressions were used to study leaf anatomy and stomatal characteristics, respectively. The lowest-yielding selection, TFM 16, had significantly greater CER (December), narrower leaves and lower boundary layer resistance (r[subscript a]). While differences among selections were not significant in April, TFM 16 tended to have higher mean CER and Tn and lower mean resistance components - total resistances to water vapor and CO₂ diffusion (ΣrH₂0 and ΣrCO₂, respectively) and stomatal resistance (rs). The highest-yielding selection, TFM 26, tended to have lower CER and Tn and higher ΣrH₂O, ΣrCO₂, r[subscript s] and r[subscript a]. Differences among selections for mesophyll resistance (r[subscript m]) were not significant. Nonsignificant differences among selections for SLW and Rd were found. CER and SLW did not appear to be closely related in this study since highly significant differences in CER were found (December) while none were found for SLW. High-yielding TFM 26 had significantly greater Le and La rates in one experiment, but differences among selections were not significant in two other experiments. Leaf width was a relatively more stable morphological character than Le or La rate and appeared to have an important influence on La rate. Leaf width, therefore, merits further consideration in forage physiology and breeding studies. A generalized description of leaf anatomy of these selections includes several noteworthy characteristics. Large contributions by bulliform cells and leaf ridging to adaxial epidermal tissue area are expected to be important in the control of leaf roll and boundary layer depth, respectively. The presence of lignified vascular bundle fibers and fiber caps may have negative effects on forage quality. Approximately half of the veins are major veins, important in long-distance transport. While mesophyll cell area comprises the major fraction of leaf cross-sectional area, large air spaces are also found within the leaf. Study of stomatal characteristics revealed that adaxial stomatal frequency exceeded that of the abaxial leaf surface by more than 3.5 times but adaxial stomatal size was only somewhat smaller than abaxial stomatal size. Consequently, adaxial stomata may be of primary importance to gas exchange when stomata are open. The lowest-yielding selection, TFM 16, had a significantly larger percentage of cross-sectional area invested in epidermal cell layers and in bulliform cell area. This latter feature, in concert with TFM 16's generally smaller abaxial stomata and lower abaxial stomatal frequency, may provide an advantage in the restriction of water loss during water-stress periods compared to the other selections. Furthermore, TFM 16's generally greater adaxial stomatal frequency may be relatively more advantageous for gas exchange when water is not limiting and may be related to its higher CER. However, TFM 16's greater investment in fiber cap tissue may represent a negative forage quality factor. Selections TFM 26 and TFK 12 had more vascular bundles than TFM 16. While TFM 26 had more minor veins, TFK 12 had more major veins and, consequently, greater phloem tissue height and width than TFM 16. However, TFM 16's significantly shorter interveinal distance may facilitate transport of photosynthate and may, therefore, be related to TFM 16's higher CER. Results of this study indicate that differences in physiological, morphological, anatomical and stomatal characteristics exist among these four tall fescue selections. Further study of the relationships among these characteristics and of their importance to forage yield and quality may suggest characteristics which can be profitably manipulated by plant breeders to bring improvement in tall fescue forage.