Heteroblastic root development in Ceratopteris richardii (Parkeriaceae)

Heteroblastic variation among leaves arising from the shoot system is a well-known phenomenon, but relatively little effort has been directed at assessing whether comparable variation occurs among shoot-borne roots in homorhizic plants. A morphological and anatomical study of shoot-borne root formation in the homosporous fern Ceratopteris richardii revealed two developmentally distinct root populations. Roots in the first population were produced once each shoot plastochron, with a single organ originating just beneath each leaf. Among the first six of these roots produced by young shoots, the root plastochron and individual growth rate, the final organ size (diameter and length), the anatomy, and the onset of lateral root initiation all varied as a function of shoot age. The positional differences observed among these organs constituted the first portion of a root heteroblastic series that continued into the adult shoot. The second root population arose from leaf bases beginning near the eighth node, with roots becoming more numerous on leaves at successive nodes. This secondary form of shoot-borne rooting behavior constituted another aspect of heteroblastic variation, but it is attributed to differences in developmental potential among leaves along the shoot and not to root heteroblasty in a strict sense. The results underscore basic differences in the morphological relationship between root and shoot systems in homorhizic ferns compared with seed plants and show that C. richardii is a useful model for investigating aspects of phase change related to rooting behavior that are not represented in flowering plants.