552 



MOSSES AND FERNS 



CHAP. 



About the time that the young sporophyte breaks through 

 the prothallium, the second leaf begins to develop. The grow- 

 ing point (Fig. 318, st) 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 2 ) arises as a slight elevation 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 other- 

 wise resembles in all respects. 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 con- 

 tinued 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 

 bundle formed from the coales- 

 cence of the bases of the bundles from the leaves and roots. In 

 all the later-formed leaves and roots there is but a single axial 

 bundle. In the leaves this is decidedly collateral in form with 

 the poorly-developed xylem upon the inner (upper) side. Ex- 

 cept for their larger size, and their having usually four instead 

 of two air-channels, the later leaves resemble in all respects those 

 first formed. 



The development of the young plant was not followed be- 

 yond the appearance of the third leaf, but it probably in its later 

 history corresponds to I. lacustris. In the latter, according to 

 Hofmeister ((i), p. 354), the opposite arrangement of the 



FIG. 319. Longitudinal section of the 

 second root, XS^s; PI, plerome. 



