290 



MOSSES AND FERNS 



CHAP. 



The growing embryo is for a long time covered by the 

 prothallial tissue, which in the upper part continues to grow 

 with it ; but finally cotyledon and root break through, the 

 former growing upward, the root bending down and anchoring 

 the young sporophyte in the mud. Owing to the large air- 

 spaces the cotyledon is lighter than the water, and always 

 stands vertically, whether the original position was vertical or 

 horizontal. In the latter case the plant appears to be attached 

 laterally to the prothallium, and the stem apex, which when 

 first formed stands almost vertically, now assumes the horizontal 

 position which it has in the older sporophyte. 



About the time that the 

 young sporophyte breaks 

 through the prothallium, the 

 second leaf begins to develop. 

 The growing point (Fig. 153,.?/) 

 now lies in the groove between 

 the base of the root and the 

 cotyledon, and its nearly flat 

 surface is at right angles to the 

 axis of the latter. The second 

 leaf (L^) arises as a slight eleva- 

 tion on the side of the stem 

 directly opposite the cotyledon. 

 From the first it is multicellular, 

 and its growth is entirely like 

 that of the cotyledon, which 

 it otherwise resembles in all re- 

 spects. Almost as soon as the 

 leaf is evident at all, a strand of procambium cells is formed 

 running from the junction of the cotyledon and first root, and 

 is continued into the second leaf as its plerome. 



The second root develops from the base of the second leaf 

 in the immediate vicinity of the common fibrovascular bundle, 

 and is formed about the time that the leaf begins to elongate. 

 A group of cells here begins to multiply actively, and very 

 soon shows a division into the initials of the tissue systems of 

 the young root. From this time the growth proceeds as in 

 the primary root, and it finally breaks through the overlying 

 tissues. 



The stem has no vascular bundle apart from the common 



Fig. 154. — Longitudinal section of the second 

 root, X525 ; //, plerome. 



