Development and Vascular Organization of the Foliar Organs of Carya cordiformis

Abstract

1. The promeristem of the stem axes of seedlings of Carya cordiformis has its origin in three successive horizontal initial units, associated at the apex of the stem's axis in the form of an inverted cone or wedge. 2. Foliar organs formed by the main axis of seedlings 3-8 weeks old consist of 6-8 scale leaves, 5-6 foliage leaves, and primordia. The transition from scale to foliage appendage is from scale leaves exhibiting a marked, although reduced, laminate structure to uni-, tri-, and five-pinnate foliage leaves. 3. Variability in leaflet number, as well as an asymmetrical formation of leaflet primordia, occurs in the foliage leaves both of seedling and winter buds; in seedlings an increase from one to five leaflets with successive leaves, in winter buds an increase from seven to nine leaflets in the first six leaves, but in later formed leaf primordia a decrease from seven to five leaflet primordia. 4. Lateral leaflet primordia in the foliar organs arise in acropetal succession, as successively developed lobes on the marginal meristems of the "upper leaf" of the primordium, with a reduced development of this portion of the primordium in the scale and first formed foliage leaves of seedlings. 5. Differentiation of the desmogen or procambial elements of the "primary" vascular bundles of the foliage leaf takes place early, and is first evident in that region of the primordium destined to become the upper section of the leaf base. The first strand appears while the primordium is in the unilobate condition, as a group of desmogen elements occupying a central position at the base of the median leaflet lobe. Other primary strands arise later, with the evolution of lateral pinnae, one beneath each of the lateral leaflet primordia. With continued meristematic activity in the basal section of each leaflet, primordium "secondary" desmogen strands arise to either side of the differentiating primary strands. Longitudinal development of these several groups of strands (both primary and secondary) is from their point of origin both acropetal and basipetal. 6. Concurrent with the origin of the primary bundles of the lamina is the differentiation of a set of vascular strands deep within the primordial leaf base. These strands (5-9 in number) differentiate acropetally only to a point of union with the primary laminar bundles. Basipetally they maintain a direct and independent course, entering the primary vascular axis to either side of the median laminar strands. Within the primary cylinder they occupy the slightly depressed regions of the vascular axis, while the laminar strands (lateral and median) mark the position of the outward protecting points or ridges of that axis. Such vascular elements, heretofore confused with and described as "cauline" bundles, account for the closely reticulate character of the primary vascular cylinder of woody forms like Carya and Quercus. 7. Cauline strands are distinguishable as units of the primary vascular axis of the epicotyl only through that embryonic period which precedes leaf formation.