Pistillate flower development in eastern black walnut

Preliminary observations of terminal bud development in eastern black walnut (Juglansnigra L. cv. Ogden) were made from midwinter through early May of 1987 using light microscopy. Flattened meristems, characteristic of pistillate flower initiation, were present in late February. Pistillate flower differentiation progressed significantly during early and mid-April, with sepal and ovule development being evident. Terminal bud swelling was clearly visible by the last week of April, and pistillate flowers were fully expanded by the first week of May. During the spring of 1988, a more detailed sequence of pistillate flower formation was recorded. In mid-April, pistillate flower meristems were clearly present and were enclosed in involucre tissue. Sepal development was also evident. By the end of April, a single pistil had emerged in the center of the meristem. As development continued, a single orthotropous ovule was formed and was surrounded by one integument. Concomitant with the growth of the ovule during early May, stigmatic regions enlarged, involutions developed, and vascular tissue became differentiated. Blooming occurred during the 2nd and 3rd weeks of May. During the 3rd and 4th weeks of May fertilized flowers with endosperm tissue were observed. In 1991, field observations of bud swell, catkin development, bud break, stem elongation, leaf expansion, and pistillate flower development were made to supplement the histological observations made in 1988. Overall, the developmental sequence of pistillate flower formation is similar to that of protandrus cultivars of English walnut (Juglansregia L.).