Leaf and Bud Initiation in the Gramineae

The rhizome and early aerial shoot of Agropyron repens Beauv. each possesses a relatively short apex bearing only a few primordia. During the growing season the apex becomes more elongated and bears more primordia, which now show smaller stepsin advancement. Later the apex elongates considerably and becomes transformed into the inflorescence. Of the axillary buds which grow out, the highest ones produce "blind" shoots, the lowest develop into shoots ending in inflorescences, and those underground give new rhizomes. 2. The shoot apex consists of three thimble-shaped layers-the dermatogen, the hypodermis, and the subhypodermis, which inclose a central core. There are probably three separate sets of initials or initial groups, one each for the dermatogen and the hypodermis and a common one for the subhypodermis and core. 3. Leaf primordia can first be detected by isolated periclinal divisions in the dermatogen cells on the side of the apex. These divisions are themselves probably preceded by similar ones in the hypodermis. Further divisions in the dermatogen and the hypodermal cells lead to the production of a crescentic protuberance. Increased activity in the region of the future mid-insertion insures the continued distichous phyllotaxis of the shoot. Lateral spreading of the divisions around the axis converts the crescent into a collar which then grows upward and produces a miniature cowl inclosing the apex and the younger primordia which have since arisen. Continued activity of the dermatogen at the free edge of the collar and then the cowl adds tissue to the tip and margins of the primordium. The internal tissue of the young leaf is derived from both the dermatogen and the hypodermis; the subhypodermis and core contribute nothing. 4. The axillary buds, which are fore-shadowed by the production of radial files of cells in the subhypodermis, emerge soon after the insertion of the leaf above has spread around from the other side of the apex. No periclinal divisions occur in either the dermatogen or the hypodermis at the position of the future bud tip, so that these two layers in the bud are derived directly from the same layers in the main shoot. The subhypodermis and core of the bud are derived from the subhypodermis of the main axis. 5. The mode of origin and development of the axillary buds should be borne in mind when amphidiploids are to be produced by the action of colchicine on seedlings or young plants. 6. It is suggested that the zonation observed in the apex is not due to any particular qualities inherent in the cells themselves but to the particular types of metabolism in these strata. This leads to a less rigid concept than that of either the Hanstein or the Breslau schools.