288 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



embryo begins to elongate, the bud with its first leaves, the cotyledons, grows erect 

 out of the apical part of the spore. The formation of the first root, a lateral root, 

 begins rather late between the foot and the suspensor; the apical cell of this root 

 originates in an inner cell of the tissue of the older segment, and the first layer of its 

 root-cap is formed by the splitting of the dermatogen which covers it into two layers ; 

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



J> 



FiG. 232. Formation of the embryo of Selaginella Martensit. A, B lower portion of the suspensor with the apical 

 cell s of the young stem ; bb the first leaves. C apical view of the preceding. D the apex only seen from above, and 

 forming two new apical cells right and left. / basal, // transversal wall ; /', //', ////, /K, V, yj', yil' the longitudinal 

 walls by which two new apical cells are formed. After Pfeffer. 



It has been already mentioned that in Pteris the plane of the apical cell of the 

 growing stem forms an angle of about ninety degrees with that of the embryo. Some- 

 thing of the same kind takes place in Selaginella, where the apical cell which lies 

 between the first two leaf-rudiments is divided by walls so disposed as to form a 

 four-sided wedge-shaped apical cell (Fig. 232 C, Z>), from which segments arise in 

 decussate pairs. In the fifth or sixth segment a second four-sided apical cell is 

 formed by a curved wall which is convex towards the first apical cell, so that a 

 vertical plane passing through these two apical cells intersects at right angles the 

 common median plane of the first leaves and that of the original two-sided apical cell. 

 Each of the two four-sided apical cells then developes into a branch of an apparent 

 bifurcation, but neither grows on in the direction of the hypocotyledonary axis ; the 

 branching therefore comes immediately over the first leaves or cotyledons. The 

 four-sided apical cells of the two shoots are however soon transformed into two-sided 

 apical cells producing two rows of segments. 



The rudiments of all the organs are formed and the first branching takes place 

 before the embryo issues from the spore. 



External differentiation. The stem is distinguished in Isoetes by the unusually 

 small amount of its growth in length, which is accompanied in this, as in the similar 

 cases of the Ophioglosseae, Marattiaceae and many Filices, with an absence of 

 branching ; no internodes are formed, and the leaves with their broad insertions are 

 arranged in a compact rosette and leave no portion of the surface of the stem 

 uncovered. The upper part of the stem which is occupied by the leaves has the shape 

 of a shallow funnel, sinking in towards the middle where the apex is (Fig. 233). 

 The considerable and permanent growth in thickness, which distinguishes the stem of 

 Isoetes from that of all other Cryptogams, is due to an interior layer of meristem, 



