X. On Isoëtes japonica, A. Br.

Abstract

Summary The trilobed caudex of Isoetes japonica consists of two distinct structures, viz. Stem and Rhizophore, to which the leaves and roots are respectively attached; but owing to the stunted growth of the plant, all external morphological differentiation between the two organs has been completely lost. Stem.—The stem-apex has the form of a conical mass of tissue situated at the base of the funnel-shaped depression in the cortex. In this protuberance no definite apical cell can be distinguished. Primary xylem, phloem, and cortex are differentiated from the primary meristem of the stem. The eauline primary vascular axis is a non-medullated monostele. Primary phloem, in which true sieve-tubes occur, surrounds the central xylem-core. An endophytic mycorrhiza is found in the peripheral cells of the primary cortex. The cambium, which arises very early from the outermost layer of the plerome, cuts off secondary cortex externally and secondary phloem internally. Sieve-tubes with sieve-areas of the typical cryptogamic type occur both in the primary and in the secondary phloem, and are continuous with those of the leaf-traces. No secondary xylem is formed in Isoetes japonica. Rhizophore.—The roots, the vascular bundles of which are collateral with usually endarch protoxylem, are arranged in acropetal series upon a distinct root-bearing organ, the rhizophore, which in this genus must be regarded as an organ sui generis. The primary growth of the rhizophore proceeds from a primary meristem situated along three radiating lines which correspond to the main fissures in the caudex. The primary and secondary tissues of the rhizophore are essentially similar to the corresponding tissues of the stem. Leaf.—The protoxylem of the collateral vascular bundle is exarch in the lamina, but becomes mesarch in the sporangial region of the leaf. True sieve-tubes occur in the phloem of the leaf. The ligule is very well developed in Isoetes japonica. It has a protective function; the young ligule envelops the younger leaves and also secretes mucilage. Systematic.—The species of Isoetes can be grouped together under two sections, Eu-Isoetes and Cephaloceraton. Isoetes occupies an isolated position amongst recent Vascular Cryptogams, and is regarded as the sole living representative of the Class Isoetales. In conclusion, we wish to express our thanks to Professor J. B. Earmer, F.R.S., for his valuable advice and kindly criticism throughout the course of this investigation.