Perspective

A large part of this book, thus far, has dealt with the typical condition in stems of gymnosperms and dicotyledons. This chapter will present interesting and important information about stem growth in monocotyledons as well as development and patterns of organization in lianas (vines) and other plants usually characterized as having “anomalous” structure. Unlike gymnosperms and dicotyledons, monocotyledons, even the largest taxa among the palms, do not produce a typical vascular cambium. Although most are characterized solely by primary growth, some palms, some members of the Liliaceae and Agavaceae, and a few other monocotyledons increase in size by secondary growth. The tissues derived from the secondary meristem are strikingly different from the secondary xylem and phloem of the gymnosperms and other angiosperms.

Primary peripheral thickening meristem

As in other plants, the activity of apical meristems of monocotyledons results primarily in an increase in length of the stems. The diameter of a palm stem does not vary greatly from the base to the most distal leaf-bearing region; thus considerable diametric growth must occur in the internodes just beneath the apical meristem, and this is accomplished by activity of the primary peripheral thickening meristem. This meristem is a rather diffuse region located in the periphery of the broad region of the stem immediately below the apical meristem. Its longitudinal extent varies in different species.