ON RHIZOCLADS IN THE OAK FERN, QUERCIFILIX ZEILANICA

Stem branches in Quercifilix arise in the axils of roots at their attachment to the parent rhizome or apically upon a new root before it has erupted from the tissues of the rhizome. In either case, they arise from the surface cells of the accessory calyptra. When the new stem arises apically, the root apex and the root cap give rise to a continuous core of procambium. The apical cell of the root becomes a tracheid, more or less retaining its original shape. The rootshoot axis thus produced is referred to as a rhizoclad. THE RHIZOCLAD, a single continuous axis with root at one end and stem at the other, is a botanical curiosity in taxa that show a sharp differentiation between roots and stems. The one described below was discovered quite by accident during a study of fern stem apices. MATERIALS AND METHODS-The plant material was obtained from cultivation. Its preparation involved the use of standard histological procedures. Staining was done with Heidenhain's hematoxylin and fast green. DESCRIPTIONLateral buds in Quercifilix (Fig. 1 ) on the rhizome replace roots in a regular system of rhizotaxy. They arise from an irregular surface meristem on the accessory calyptra of certain roots. The accessory calyptra is that covering of an endogenous root tip that arises from the epidermis and outer cortical layers of the parental organ, in this case rhizome. Ferns in general produce accessory calyptras that are ruptured eventually by greater growth of the root apex below. The amount of accessory calyptral growth determines the extent of the collar of torn tissue present at the base of the older root. It might be guessed that cells external to an endogenously produced root apex divide anticlinally as a response to physical pressures from below, an interpretation perhaps valid in certain instances, but at the surface in Quercifilix the surface meristem of the accessory calyptra is composed of a number of small cells and is often somewhat sunken and overgrown from the sides. ] Received for publication 29 January 1985; revision accepted 30 April 1985. 2 Present address: Rt. 3, Box 188, Picayune, MS 39466. Certain root tips are broader than others, even before they elevate the surface of the rhizome. The broader root tips grow, and their accessory calyptras grow with them. When they protrude at the surface they expand laterally, become often near-spherical and quite green (Fig. 5). Much of the lateral tissue of these expanded bodies is derived from the accessory calyptra, and, in fact, such root tips are meristematically quite inactive with activity restricted to the accessory calyptra. The specimen in Fig. 5 is actually a nearly completely formed rhizoclad. It shows a stem apex overlapped by scales at the top and a procambial connection between the root stele and the stem stele. The root stele curves sharply out of the plane of section. A younger rhizoclad is shown in Fig. 3, where only the central core of tissue appears. A welldifferentiated stem apex is at the distal end of the rhizoclad (right) and some cells (ca. 10-12) below is the apical cell of the root. Differentiation of procambium in the root had already progressed so far that the root apical cell was quite procambial in nature. In older specimens in which the stelar tissues are mature, the apical cell can usually still be recognized as a tracheid. The specimen in Fig. 2 is more advanced than those in Fig. 3 and 5. The stelar tissues of this rhizoclad are well laid out as procambium. The narrow root stele appears within the tissues of the parental rhizome (only cortex of which is shown) and the expanded stele of the new rhizome on the right in the distal end of the rhizoclad. The scales covering the new stem apex were removed in order to identify the structure before fixation. The rhizoclad in Fig. 1 is several times longer than the one of Fig. 2. It is approximately one-