Seedling growth and root contraction in the soap plant, Chlorogalum pomeridianum (Liliaceae)

Radicles and adventitious roots of the soap plant are contractile and through their activity, mature bulbs of this species are buried to depths of 20-30 cm. Experiments were performed to determine rates of contraction and displacement of the shoot apex resulting from activity of the contractile radicle and the first several adventitious roots. The average displacement was 23.2 mm over a 10-wk period and 63.8 mm over 29 weeks. Small glass beads and abortive seeds served as controls and showed no displacement through the soil column. Measurements from longitudinal and transverse sections of contracted and uncontracted portions of radicles revealed average increases of 26-64% in radial dimensions and 40-56% decreases in longitudinal dimensions of inner and middle cortical cells (excluding the endodermis) following contraction. Cells of the outermost cortex (excluding the exodermis) decreased in average longitudinal dimensions by 18-26% before becoming distorted and collapsed as contraction was completed. Average volumes of innermost cortical cells decreased by 15-54%, while two or three cell layers of the middle cortex, adjacent to collapsed outer cortical cells, increased in volume up to 75%. These middle cortical cells are identified as the "active" cells which, by their growth, are responsible for the shortening of the root. Throughout the process of contraction, the stele remains straight and undistorted, although the closer spacing of tracheary element secondary wall thickenings following contraction suggests longitudinal compression ofthe stele. The average number of cortical cells per transverse section does not differ in contracted and uncontracted roots and no evidence is found to support the "interdigitation" hypothesis of root contraction. However, reorientation of middle cortical cell expansion may be the mechanism of root contraction in Chlorogalum pomeridianum. FOLLOWING SEED GERMINATION at or near the soil surface, shoot apices of many geophytic plants are buried at definite levels in the ground through the activity of contractile roots. Several types of root contraction have been described. Root contraction in many monocotyledons is associated with tangential and radial enlargement of inner cortical cells and the collapse of outer cortical parenchyma (Arber, 1925; Gravis, 1926; Wilson and Honey, 1966; Chen, 1969). Thisis usually, but not always (e.g., Freesia, Ruzin, 1979), accompanied by longitudinal shortening of inner cortical cells. In Oxalis, Brodiaea, and Zamia, transverse discs of cortical cells collapse and are crushed longitudinally by layers of turgid cells above and below (Thoday, 1926, 1931; Smith, 1930; Stevenson, 1980). A third pattern of contraction has been reported in the fern, Botrychium (Stevenson, 1975), in which lysigenous cavities ' Received for publication 1 1 December 1982; revision accepted 25 July 1983. Partial support for this work was provided by NSF Grant PCM-8202756. formed in the cortex and subsequently collapsed, resulting in a shorter root. There were three objectives of the present investigation: 1) to confirm and expand upon Rimbach's (1902) observations on the early development of Chlorogalum pomeridianum seedlings; 2) to quantify the amount of contraction occurring in young seedlings and assess the rate of shoot apex movement under controlled conditions; and 3) to describe the cellular changes accompanying root contraction in this species. The results are discussed with reference to two modern hypotheses for the mechanism of root contraction, i.e., cell interdigitation (Sterling, 1972), and growth reorientation (Wilson and Honey, 1966). MATERIALS AND METHODS-Mature seeds of Chlorogalum pomeridianum (DC.) Kunth were collected from natural populations in northern CaliforniaS stored at 0 C for a minimum of 3 months, and germinated on moist vermiculite. Seedlings were grown in vermiculite in either 12-cmplastiopotsor3 x 3 x 25-cm(width x breadth x height) clear Plexiglas columns