288 MOSSES AND FERNS chap. 



embryo. Beneath are two layers of cells concentric with the 

 epidermis. From the inner one arises the initial cell (or cells ?) 

 of the plerome, which soon becomes well defined and connected 

 with the primary strand of procambium in the axis of the 

 embryo. It is quite possible that here, as in the older roots, 

 a single initial cell is present in the plerome, but this is not 

 certain. The layer of cells immediately below the primary 

 epidermis is the initial meristem for all the tissues of the root 

 except the plerome. The primary epidermis later divides into 

 two concentric layers which take no further part in the growth 

 of the root except as they join the outer layers of the root-cap. 

 From the layer above the plerome initial, additions are 



Fig. 152. — Median longitudinal section of an embrj^o of the same species shortly before the cotyledon 

 breaks through the prothallium ; lettering as in the preceding, X300. 



made at regular intervals to the root -cap, and these layers 

 remain one cell thick, so that the stratification is very "marked. 

 At the apex of the root there is no separation of dermatogen 

 and periblem, which are first differentiated back of the apex. 

 The primary xylem consists of very delicate spiral tracheids 

 formed at the base of the root at the same time that the first 

 ones appear in the leaf. 



The foot increases much in size as the leaf and root develop, 

 and its superficial cells become much enlarged and encroach 

 upon the large cells of the prothallium, whose contents are 

 gradually absorbed by it. 



The cotyledon is at first composed of compact tissue, which 



