321 Morphological Shoot Apex Changes During Inflorescence Development in Heliconia rostrata Ruiz & Pavon

The colorful and pendulous inflorescence of Heliconia rostrata Ruiz & Pavon terminates an erect and herbaceous-musoid axis of a sympodial rhizome system. Each hapoxanthic axis bears a variable number of leaves (5 to 10) subtending the inflorescence. The number depends on the time between shoot emergence and flowering stimulus. Inflorescence initiation and development occurs without external evidence of this process until the inflorescence emerges from the pseudostem. The morphological changes occurring at the terminal shoot apex of the H. rostrata as it changes from vegetative to the flowering stage are described and illustrated by photomicrographs in this paper. The anatomical sections reveal that the apex on vegetative phase is domed, and a maximum of four furled leaves including one leaf primordium can be observed surrounding it. The growth of the leaf primordium is highly synchronized with growth of the most recently formed leaves. With the transition to inflorescence development, more primordia are observed on the apex, which ultimately give rise to the bracts. Except for the first sterile bract, a cincinnus primordium (flower cluster) is detectable in the axil when the next bract begins to develop. Flower differentiation on the cincinnus begins when many bracts are well-developed. The increase of longitudinal height on the internodes is among the first detectable morphological changes in the apex. Under inductive conditions, the transition to the reproductive stage is achieved early in plants with three or more unfurled leaves. The reproductive plant status is easier to detect under the microscope when the inflorescence has at least three bracts.