150 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[30. 



c. Growing-points with a group of common initial cells occur in 

 certain Pteridophyta (e.g. in the stems and roots of Marattiaceae, 

 of Osnmnda sometimes ; stems of Lycopodium, Isoetes, some 

 Selaginellas). In these cases, there is a group of frequently four 

 cells which are the common initials of the tissue- systems. The 

 general relations of the tissue-systems are here essentially the 



P Pb 



FIG. 115. Median longitudinal section through the apex of the root of Pteris cretica ; t 

 apical cell; fc initial segment of dermatogen; fc, outermost layer of root-cap ; p wall mark- 

 ing limit between the plerome P and the periblem P6 ; c cell marking the inner limit of 

 the outer cortex. (After Strasburger : x 240.) 



same as in those forms in which the growing-point has a single 

 apical cell : in these cases, as in those, there is no properly differ- 

 entiated dermatogen in the stem, and consequently there is no layer 

 which is morphologically equivalent to the epidermis of the stem 

 in the Phanerogams. 



