Ontogeny and Differentiation of Sclereids in Rauwolfia

Abstract

An interesting anatomic feature of Rauwolfia is the occurrence of a remarkable type of sclereid in the stem and root. The initials of the sclereids in the stem arise in the ground tissue element of the pith in a region between 50 and 70μ below the surface of the shoot apex. This region of the shoot remains surrounded by a whorl of either 3 or 4 leaves. Sclereids initiate in succession in association with each whorl of leaves. Thus, the sclereids are restricted to the nodes. The sclereids in the stem arise as a primary element of the shoot from the ground tissue of the pith. In the root, they differentiate from the cells of the phelloderm and are secondary in origin. Morphologically, the sclereids in these 2 organs are basically the same, except that the sclereids in the stem are larger in size than those in the root. A solitary cell, or 2 to several cells in a longitudinal cell file (originated from a single mother cell), may differentiate into sclereid initials. The growth of the sclereids through relatively compact ground tissue of the pith is possibly accomplished by a combination of gliding growth and apical intrusive process. The sclereid initials grow rapidly and force their way between the parenchymatous cells. As a result, the neighboring cells lose their original surface contacts. Sclereids increase in size rapidly, and, therefore, very enlarged sclereids with thin primary walls may be observed in the second node. They mature progressively in basipetal direction in the subjacent nodes. In the fifth or sixth node, mature sclereids with massive secondary walls are most common. The secondary walls of sclereids contain much lignin as determined by the phloroglucinol‐HCl test. The walls of sclereids at maturity show a variable number of lamellae ranging from 10 to 15 in the lateral walls. A remarkable feature of the sclereids is their canal‐like pits in the secondary walls. Two adjacent pits may coalesce uniquely to form a Y‐like configuration directed centrifugally from the lumen of the sclereids. The sclereids are ventrically symmetrical, joined end‐to‐end by their transverse walls like 2 superimposed young fibers.