PTERIDOPHYTA 



255 



seen in Struthiopteris, where numerous stolons develop from the old 

 leaf-bases. 



The Stem 



The growth of the stem, in the typical Ferns, is due to the division 

 of a single tetrahedral apical cell, which in unbranched stems is the 

 direct descendant of the orginal stem-quadrant of the embryo. The 

 segmentation of the apical cell is usually slow, and it is generally 

 impossible to determine the exact relation of the leaves and lateral 

 branches to the primary segments of the apical cell. 



Early divisions in the young segments separate a central cylinder, 

 in consequence of which the vascular bundles and pith (when pres- 



A. 



D 



FIG. 219. A-C, Adiantum emarginatum. A, longitudinal section of stem-apex 

 (X 25) ; x, apical cell; I, young leaf. B, apex of the same (X 180). C, cross- 

 section of the stem-apex (x 180). D, young leaf of Struthiopteris Germanica, 

 showing apical cell, . 



ent) are delimited from the cortex. In case there is a single axial 

 bundle, the stem is "monostelic" ; if more than one vascular bundle 

 is present, the stem is " polystelic." The ground-tissue may remain 

 permanently parenchymatous, or it may develop sclerenchyma, which 

 is characteristic of the sterns of many Ferns. The typical scleren- 

 chyma (Fig. 221) is made up of cells with very thick striated and 

 pitted walls of a golden or dark-brown color. 



Vascular Bundles. The vascular bundles of the stem, in most 

 Ferns, form a hollow network within which lies the pith. The 

 spaces between the bundles are the " foliar-gaps," and it is at these 



