350 FERNS 



cells which constitute the root-cap (r. cp\ In fact the apical 

 cell of the root divides not only by planes parallel to its 

 three sides, but also by a plane parallel to its base, and in 

 this way produces not only three series of segmental cells 

 (seg. c] which afterwards subdivide to form the apical 

 meristem, but also a series of cap-cells (cp. c] which form a 

 protective sheath over the tender growing end of the root as 

 it forces its way through the soil. 



Roots are also peculiar in their development. Instead of 

 being like leaves, prominences of the superficial tissues of 

 the stem, they arise from a layer of cells immediately ex- 

 ternal to the vascular bundles, and in growing force their 

 way through the superficial portion of the stem, through 

 a fissure in which they finally emerge. They are thus said 

 to be endogenous in origin while leaves are exogenous. 



The nutrition of ferns is carried on in much the same 

 way as in mosses (see p. 336). Judging from the analogy of 

 flowering plants it would seem that the ascending current of 

 water from the roots passes mainly through the xylem of the 

 vascular bundles, while the descending current of nitrogenous 

 and other nutrient matters for the supply .of the colourless 

 cells of the stem and roots passes chiefly through the phloem 

 and especially through the sieve-tubes. The absorption of 

 water is effected by the root-hairs. 



In the autumn there are found on the under surfaces of 

 the leaves brown patches called son, differing greatly in 

 form and arrangement in the various genera, and formed of 

 innumerable, minute, seed-like bodies, the sporangia (Fig. 

 84, A), just visible to the naked eye. Each sorus or group 

 of sporangia is covered by a fold of the epidermis of the 

 leaf, called the indusium. 



