PREVIOUS STUDIES of the meristem in the Gramineae have given little attention to varietal differences and the influence of temperature in early development of the shoot. This study was undertaken to determine what ontogenetic differences are present in the early development of four varieties of Avena sativa and what effect temperature may have on the development of each variety. Four varieties including a brachitic dwarf were grown at 160C. and 280C. The size, structure, and form of the mature oat plant is controlled by the pattern of differentiation from the meristem during formative stages. Therefore a study of the meristem should be of value in understanding the morphology of the mature plant. All the leaves are initiated during the first 3 weeks in the four varieties growing at both high and low temperatures. At the high temperature, foliage development is somewhat more rapid and cell division, resulting in initiation of each new leaf primordia, is observed 2 or 3 days earlier than at the low temperature. The 3-week period of leaf initiation is considered the vegetative or formative period. Panicle differentiation follows rapidly after the initiation of all leaf primordia. At 28?C. panicle formation is inhibited. Thus environmental factors in the early stages have a determining effect on the mature plant. Varietal differences which are prominent in the mature stages are also apparent in the structural development of the shoot. MATERIALS AND METHODS.-Four varieties of Avena sativa were grown at constant temperatures of 160C. and 280C. in the Plant Pathology greenhouses at the University of Wisconsin during the period from November to January, 1945-1946. No artificial light was used. The plants were grown in 8-in. clay pots, fifteen grains being sown in each pot. The seed was obtained from the Cereal Division, Central Experimental Farm, Ottawa, Canada. The four varieties studied were Beacon, Dasix, Roxton, and Trelle's Dwarf. They were chosen to represent different growth habits and were known to vary in height and diameter of culms, in leaf and root development, and in maturity rates. The two early maturing varieties, Dasix and Beacon, provide a contrast in culm, leaf, and root development. Dasix has taller but smaller culms, narrower leaves, and less extensively developed adventitious roots than Beacon. Roxton is a tall, late variety with medium-sized culms. The brachitic dwarf variety, 1 Received for publication April 23, 1948. Part of a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science at the University of Wisconsin. Acknowledgments are gratefully made to Dr. Emma L. Fisk for her helpful guidance throughout the study, to Dr. J. G. Dickson for his interest and suggestions, and to Mr. Eugene Herrling who took the photomicrographs. Trelle's Dwarf, usually grows only 18-24 in. high. The dwarf characteristic is apparent in internodal length but not in the size of leaves or diameter of culm. Under greenhouse conditions the plants of Beacon and Trelle's Dwarf remained erect while those of Roxton and Dasix lodged. Two plants from each variety, grown at two temperatures, were collected at 2or 3-day intervals. The last collection was made at the end of 58 days when differentiation of the panicle had begun. With the aid of a binocular dissecting microscope the older leaves were removed to assure more rapid and satisfactory fixation. In younger plants the coleoptile node and attached stem tip were fixed while in older plants only the portion above the third node was used. Embryos from soaked grains of each variety were fixed after being removed from the endosperm. Karpechenko's modification of Navashin's chromo-acetic-formalin fixative gave good results. The material was washed, dehydrated with etbyl alcohol, and run into paraffin by the cedarwood oil method. Sections were cut longitudinally at 10 . Most sections were stained with a weak Haidenhain's iron-alum haemotoxylin and safranin stain (Esau, 1944). Some sections were stained with fast green and safranin (Cross, 1937) and others with tannic acid, zinc chloride, orange G, and safranin (Sharman, 1943). OBSERVATION AND DISCUSSION.-Stages of growth from germination to panicle initiation.-The mature oat plant develops from an embryo in which the cotyledon, coleoptile, two foliage leaves, and the seminal root system are differentiated. The root system of the embryo includes the primary root, ensheathed in a coleorhiza, and usually one pair of lateral, seminal roots which are located below the cotyledonary node. Provascular strands are differentiated in the epicotyl and supply the cotyledon, coleoptile, and first leaf (Avery, 1930). Elongation of the first internode begins at germination and will eventually raise the second or coleoptile node to the ground level. One of the first changes observed at the time of germination is the initiation of the fifth leaf or the third leaf above the coleoptile by periclinal divisions of the dermatogen layer near the tip of the epicotyl (fig. 9). Throughout this study the cotvledon will be considered the first leaf and the coleoptile the second leaf. In median sections cut from embryos removed from grains soaked for 24 hr., recent division of the dermatogen layer indicates that the first step in initiation of a new leaf occurs very soon after planting. Mature oat plants usually have eight leaves, or six leaves above the coleoptile. Four leaves are initiated after germination and thus during the vegetative period of growth four plas