THE ROOT APEX OF LINUM

Several developmental stages of primary roots of 13 species of Linum were studied. The basic pattern of root apex organization in three species consisted of a single tier of cortical initials. Ten species had a basic pattern of two tiers of cortical initials. Variations, manifested by either an increase or a decrease in the tiers of cortical initials, were observed in some roots in those species that had a basic pattern of two tiers of cortical initials. Although these variations were interpreted as being ontogenetic, there was no total reorganization of the root apex. There was anatomical and cytological evidence that a quiescent center is established in Linum roots. ROOT APEX organization in Linum ustatissimum L. varies from the common pattern in roots of dicotyledons. Janeze vski (1874) and Eriksson (1876) described two layers of cortical initials rather than a single one which is the common condition reported for dicot roots (IKroll, 1912). In L. usitatissimum the outer layer is continuous with a subepidermal layer and is independent of the remainder of the cortex. Flahault (1878) described the same pattern for L. grandiflorum Desf., and Crooks (1933) confirmed the two-layered condition for L. usitatissimum. All of these investigators also found a layer of rootcap-epidermis initials and a group of central cylinder initials. There has been only one previous anatomical study of the root apices in several species within a genus, that of Cassia (Hayat, 1963). More recently, genera of the Convolvulaceae (Seago, 1966) and the Malvaceae (Camp, 1966) have been investigated. Because Linum is a rather large genus (approximately 200 species), the present investigation was undertaken to determine if the root apex organization described for L. usitatissimum and L. grandiflorum is found in other species of the genus. Furthermore, it was anticipated that a structural pattern characterized by a double layer of cortical initials is one in which certain ontogenetic changes could be readily detected if they occurred. Attention has been givein to this aspect of root tip organization by von Guttenberg (1947) and his students (von Guttenburg, Burmiester, and Brosell, 1955) who have noted developmental changes in elongating roots of Helianthus and other genera. MATERIALS AND METHODs-Thirteen herbaceous species of Linum were used in this study. 1 Received for publication 19 Janiuary 1968. Based on a thesis submitted to the Graduate School of Miami University in partial fulfillment of the requirements for the Master of Arts degree by the first author. The assistance of Dr. C. MI. Rogers, Wayne State University, who supplied seeds of two species, is gratefully acknowledged. The embryo and root apex materials were obtained from seeds and germinated seedlings. Prior to germination seeds were treated with Spergoni (tetra-chloroparabenzoquinone) to inhibit fungal infection. The seeds were then placed on moistened filter paper in petri plates. Some species required chilling, whereas others needed soaking and washing with water to break dormancy. Most species germinated promptlv without treatment. Germination and subsequent seedling growth were carried out in the dark at room temperature (21-26 C). Upon emergence of hypocotyls the seeds were placed between moistened sheets of filter paper. The filter paper was then rolled and placed upright in beakers. Petri plates were placed over the beakers in order to maintain high humiditv. This procedure insured straight roots which were free of detritus. All glassware and filter paper were autoclaved before use. At harvesting apices were removed, killed, fixed in Craf III (Sass, 1961), and segregated according to length. Dehydration was carried out in a graded series of tertiary butyl alcohol, and specimens were embedded in Tissuemat. Serial longitudinal sections were cut at 4 or 5 p depending on the diameter of the root tip. Because of the small size of the roots, difficulty was experienced in obtaining median sections, and approximately 70% of the sectioned roots were discarded, However, the observational advantage of having the apical organization located within a minimal area compensated for the technical problems encountered. Sections were stained in safranin and fast green (Sass, 1961). Terminology-Various terms have been used to describe the meristematic tissues which comprise the root apex. Esau (1943b) reviewed the terminology which has been used to describe the primary tissues and tissue systems. Many workers have applied Hanstein's terms-dermatogen, periblem, and plerome-to the epidermis, cortex, and central cylinder in the meristematic root