XI. The Anatomy of some Sapotaceous Seedlings.

Summary 1 A swollen zone in the hypocotyl, corresponding with a band of secondary thickening, or with lignification of the pith, or with both, is seen in ten out of the fourteen species examined. Phloem is well developed. Other geophilous features are prominent in Bumelia tenax. 2 The vascular system of primary root and hypocotyl is typically tetrarch and corresponds with two bundles from each cotyledon without change of position. In Mimusops Schimperi, however, there is a hexarch root, and Bumelia tenax and two species of Palaquium are variable and anomalous, tending to be hexarch. The occurrence of the hexarch type might lead one to suspect that a central cotyledonary trace had aborted in the tetrarch type, as described in some species of Diospyros by Mr. Herbert Wright*. But in the tetrarch types of which I had seeds I could not discover, even in the embryo, any sign of lignification in the position of a central trace. The midrib of the cotyledon was formed as indicated in Pl. 26. fig. 10. The primary vascular bundles lie typically parallel to the cotyledonary plane, but are not intersected by it. 3 Vasculsr system of the cotyledons:—The first differentiation of xylem is seen at the tip of the cotyledon and progresses downwards through the node. The two strands of metaxylem belonging to each double vascular bundle remain separate and are seen halfway between the cotyledonary node and the root-apex to be placed each side of the xylem of the primary root-strand. Protoxylem-strands of the cotyledon, if double above, fuse at the node and occupy a median position. In Bumelia tenax the protoxylem strands of the cotyledon are single and occupy a median position between the members of a double group of metaxylem. They are completely isolated by parenchyma in Payena Leerii, Bumelia tenax, and Chrysophyllum Cainito. 4 Root system:—Lignification progresses from the node downwards, dying out gradually near the root-apex. The continuity of the protoxylem is sometimes interrupted for short distances, probably owing to stretching during growth. 5 Stem system:—Differentiation of xylem begins at the node and progresses downwards. In Bumelia tenax there is marked lack of continuity. The small spiral and annular tracheids belonging to the traces of the first internode do not reach to the node. This break is perhaps only an exaggeration of what is seen in the protoxylems of the root and to be accounted for by the rapid elongation of the shoot (see Bumelia tenax, fig. 11 b,). As in Diospyros the foliar traces fork and spread out at the node in this species, and are represented by a reduced number of bundles above and below the node. 6 Accessory system of the root:—In Bumelia tenux we meet with a number of strands not primary, which I have called the accessory system of the root. The strands which constitute this system do not correspond in number and position with the foliar traces which appear at the node. At the stages of development observed in the seedlings and seeds described the separateness of differentiation obtaining in these different vascular systems is very marked and clear. The impression derived from a study of these Sapotaceous seedlings is that, while there is a characteristic type of anatomy for the order, it is subject to adaptive variations as to number of strands primary and otherwise; but is constant as regards their structure, which in the primary strands is always dual.