Anatomy and Histology of the Pineapple Inflorescence and Fruit

The structure of the inflorescence, the flower, and the fruit of the pineapple, Ananas comosus (L.) Merr., is described and interpreted. 2. The collective fruit is composed of numerous fruitlets and floral bracts which are connate with one another. Each hermaphroditic flower is trimerous in pattern and subtended by a prominent bract. 3. An important feature of the tricarpellate inferior ovary is the presence of septal glands which are, ontogenetically, the result of the incomplete fusion of adjacent carpel margins. The nectary duct, terminating each gland, provides a natural outlet from these glands into the blossom cup at the apex of the ovary. The glandular tissues show a definite transition from the secretory to the non-functional stage. The nectary duct and blossom-cup linings are transformed into an impervious, protective layer by combined processes of lignification, suberization, and formation of a periderm-like layer, thus protecting the fleshy tissues from the outside. 4. The stylar canals, which are open channels before anthesis, are closed by at least two methods: (a) mucilaginous plugging and (b) cellular occlusion by tylosis-like cells. 5. Two processes contribute to the increase in size of the fruit: cell multiplication in stages up to flowering and cell enlargement in the older bud to ripe-fruit stage. Cells of the internal tissues increase rapidly in size while the fruit is maturing, become fully distended upon ripening, and begin to show shrinkage when fully ripe. The overripe stages show softening and some disintegration of the walls and occasional cellular break-down; dissolution, however, does not take place until microbial decomposition sets in. 6. Morphologically, the pineapple fruit is formed by the complete fusion of the fruitlets of an inflorescence and their subtending floral bracts to one another and to the inflorescence axis. This fusion has taken place during phylogenetic development. The flowers have typical inferior ovaries with the normal vascular supply for such ovaries of the appendicular type. Fusion of traces to the various organs has occurred whenever these bundles lie in the same radii, as is to be expected in an inferior ovary which is the result of maximum adnation (8). Fusion, likewise, has occurred between the lateral bundles of the subtending bract and some of those of the sepals. The bracts are intimately fused to their subtended flowers and to adjacent bracts. The bulk of the noncarpellary tissue between the ovaries is composed of sepal and bract tissues.