INITIATION AND ONTOGENY OF THE MICROSPORANGIATE CONE IN CUPRESSUS ARIZONICA IN RESPONSE TO GIBBERELLIN

Cupressus arizonica, a member of the Cupressaceae, was induced to produce pollen cones in response to gibberellin treatment. All apices remained vegetative during the first 17 days of treatment. At this time many lateral vegetative apices began to undergo a transition to the reproductive state. The transition was marked by changes in apical zonation characterized by increased mitotic activity primarily in the subapical mother-cell and peripheral zones. Also, precocious initiation of branch meristems occurred much higher on the shoot apex than before the transition period. About 22 days after the initial treatment, most apices became distinctly reproductive. The reproductive apex has a zonation pattern similar to the branching apex but is shorter and wider and quite distinct from the vegetative apex. The small subapical mother cells and cells of the peripheral zone form a continuous mantle of mitotically active cells with prominent nucleoli. This mantle encloses a very broad pith region which differentiates nearly to the summit of the apex. Microsporophyll and leaf initiation are similar and the protoderm of the apex remains discrete and does not contribute to deeper tissues. Sporangia do not originate from superficial cells of the microsporophyll. After all microsporophylls are initiated the reproductive apex becomes inactive. A discussion concerns the morphological implication in the origin of the foliar structures and of the similarity of C. arizonica to many angiosperms in the transition of the apex from vegetative to reproductive.