ISOETES JAPONICA, A. BE. 



337 



organ of the plant \cf. Lang (20), p. 784], aRalogous to the swollen basal region of 

 the stem of Selaginella sinnosa [Bruchmann (8)], or of Fleuromeia [Solms-Laubach, 



(30]. 



We assume, however, that owing to its extremely stunted growth, the plant has 

 completely lost all external morphological differentiation into stem and rhizophore. 



Apical Meristem and Development of the Phimae-y Tissues of the S 



The question as to the exact nature of the apical meristem of the stem of Isoetes has 

 given rise to much discussion in the past. 



The first botanist to investigate this problem was Hofmeister (16), who concluded 

 that the tissues of the stem are derived from the segments of a single apical cell, the 

 septa of which are turned towards the furrows of the stem. This apical cell was found 

 at the apex of a blunt cone of cellular tissue situated at the base of the fannel-shaped 

 depression in the upper surface of the cortex (16, pi. li. fig. 4). In arriving at this 

 conclusion, Hofmeister was probably influenced by the prevailing conception of a single 

 apical cell at the apex of the stem of all Vascular Cryptogams. 



In the year 1874, this question was discussed in papers by. Hegelmaier (15) and 

 Bruchmann (7) respectively. The former maintained that the apical meristem of the 

 stem of /. velata and of I. Durieui consists of an actual apical-cell surface (Scheitel- 

 zellenfiache}, and that the cells situated near the centre of this superficial layer of 

 meristem give rise to the mother-cells of the axile woody mass, hence they may be 

 regarded as plerome initials. The more peripheral cells of this meristematic layer, by 

 repeated divisions in an oblique plane, give rise to the primary cortex and to the leaves. 



Bruchmann, as a result of his investigations on I. lacustris, arrived at a somewhat 

 similar conclusion. He states (7, p. 576), that " Der Stamra wachst durch Urmeristem- 

 Initialen, die anfangs nnr aus wenigen Zellen bestehen." In other words, the actual 

 apical meristem is confined to a group of a few cells, which in older plants forms a slight 

 elevation near the centre of the superficial cells of the stem apex. No definite apical 

 cell was observed, but these cells (Urmeristem-Initialen), which may be distinguished 

 from the other superficial cells by their denser protoplasm and by a slight difference in 

 size, divide tangentially, giving rise to an underlying group of meristematic cells, from 

 which both the central mass of xylem and the tissue of the cortex are produced. 



In the first edition of Ids ' Traite de Botanique,' Van Ticghem, referring to the growth 

 of the stem of Isoetes, remarks {33, p. 1294) that " La tige croit, comrae celle dcs 

 Lycopodes, par un groupe de petites cellules meres ; mais ici Ton n'observe dans ce 

 groupe aucune differentiation d'initiales propres." But, in a later edition of the same 

 work (34, p. 1429), the following sentence bearing on this subject occurs, " La tige croit, 

 comme celle des Lycopodes, par une seule petite cellule mere." 



CampbeU (10, p. 249) states, in reference to the very young sporophyte, that "the 

 growing-point of the stem is a nearly fiat area, whose surface is nearly at right 



angles to the axis of the leaf." 



3c2 



