4o6 VASCULAR CRYPrOGAMS. 



afterwards between the foot and the suspensor. It is lateral, and its apical cell is 

 formed from an inner cell of the older segment ; but the first layer of its root-cap 

 originates from the splitting into two layers of the overlying dermatogen ; the 

 later layers of the root-cap arise from the apical cell of the root itself. 



It has already been mentioned that in Pteris and Salvinia the position of the 

 apical cell of the growing stem is placed at an angle of about 90° with respect to 

 that of the embryo. Something of the same kind occurs in Selaginella ; the apical 

 cell which lies betw^een the rudiments of the first two leaves is divided by walls in 

 such a manner that a four-sided apical cell is formed (Fig. 304 C, D), the segments of 

 which arise in decussate pairs. In the fifth or sixth segment a second four-sided 

 apical cell is now formed by a curved wall with the convexity turned towards the 

 primary apical cell, so that a longitudinal section through the two apical cells cuts 

 at right angles the common median line of the first leaves, and that of the original 

 two-edged apical cell. Each of the two four-sided apical cells now developes into 

 a branch of the first dichotomy ; but neither of the segments continues to grow in 

 the direction of the hypocotyledonary segment ; the division, therefore, takes place 

 immediately above the first leaves or cotyledons. The four-sided apical cells of 

 the two rudimicnts of shoots are, however, soon transformed into two-sided apical 

 cells each forming two rows of segments \ 



The first formation of all the organs and the first dichotomy always take 

 place before the protrusion of the embryo from the spore. 



The External Differentiation is very various in the different genera of Lycopo- 

 diaceae, if the habit of the mature plant is taken into account ; but they agree in a 

 few points of great morphological importance. The leaves, different as they appear 

 in other respects, are always simple and unbranched, and are penetrated by only one 

 fibro-vascular bundle ; the branching at the end of the shoots and roots is always 

 dichotomous, and the dichotomies succeed one another (with the exception of the 

 older states in Selaginella) in planes at right angles. The roots of Lycopodiacese 

 are, at present, the only ones knowm to dichotomise in the whole vegetable king- 

 dom^. The difference of habit depends especially on the relative size of the leaves 

 and on the different rapidity of the growth of the stem in length. One extreme 

 is afforded in this respect by the genus Isoetes, with its extremely short un- 

 branched stem, growing scarcely at all in length but much in breadth, its dense 

 rosettes of leaves, which are of considerable and often of very great length and 

 the number of which is often very large, and its numerous roots ; the other extreme 

 occurs in Psilotum, where the stem regularly dichotomises, remains slender, grows 

 much in length, but forms only very small leaves and no true roots at all. In 

 Selaginella and Lycopodium the leaves are not large but nevertheless strongly 

 developed, and the repeatedly dichotomising branches are densely covered with 

 leaves, and produce numerous roots in acropetal succession. Very different from 



* The newwedge-shaped apical cells of the two first branches lie parallel to the primary 

 apical cell of the embryo, as also do the apical cells of the succeeding branch. The second and 

 succeeding planes of dichotomy therefore cross the first at a right angle, but only at the outset ; 

 since, by a twisting of the first branches, their dichotomies come into the same plane as the first. 



^ According to Reinke, however, some adventitious roots of Cycadecc do dichotomise. 



