474 



VASCULAR CRYPTOGAMS. 



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

 cell, which lies between the rudiments of the first two leaves, is divided by walls in 

 such a manner that a four-sided apical cell is formed (Fig. 332 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-sided apical cell. Each of the two four-sided apical cells now developes into 

 a branch ; but neither of the branches continues to grow in the direction of the 

 hypocotyledonary axis ; the branching therefore takes place immediately above 

 the first leaves or cotyledons. The four-sided apical cells of the two rudimentary 

 branches are soon transformed into two-sided apical cells each forming two rows 

 of segments. 



Fig. 333.— Longitudinal section oilsoetes lacusiris at right angles to the furrows of the stem ten months old (after Hofmeister), 

 5' stem, b^—l^ leaves, >-i— ;-io roots (X 30) ; the ligula of the two developed leaves is shaded. 



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

 place before the protrusion of the embryo from the spore. 



External Differentiation, The Stem is distinguished in Isoetes^ as has already been 

 mentioned, by its extraordinarily small growth in length, with which is connected, in 

 this as in other cases (Ophioglossese, Marattiaceae, and many Ferns), an absence of 

 branching ; no internodes are formed, the leaves with broad bases of insertion con- 

 stituting a thick rosette, without leaving between them any surface of the stem bare. 

 The upper region of the stem, which is furnished with leaves^ has the form of a 

 shallow funnel, depressed in the centre or apex (Fig. 333). The long-continued 

 increase in thickness, which distinguishes the stem of Isoetes from that of all other 

 Cryptogams^, is brought about by an internal layer of meristem, surrounding the 



^ Compare what has been said already about Bofrychium and what follows about Lepidodendron. 



