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 lacustris at right angles to the furrows of the stem ten months old (after Hofmeister), 
S stem, h^—b^ leaves, r^—r'^o 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 Ste7n is distinguished in Tsoetes, as has already been 
mentioned, by its extraordinarily small growth in length, with which is connected, in 
this as in other cases (Ophioglossea?, 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 aheady about Bolrychium and what follows about Lepidodendron. 
