52 STRUCTURAL BOTANY 



find that the cell - walls of the inner zone are not 

 uniformly thickened ; at the points opposite the two 

 ends of the xylem-plate, the cells remain comparatively 

 thin -walled so as to leave a free passage, through which 

 the water absorbed from the soil can reach the wood, 

 and thus pass upwards to the stem and leaves. At 

 the exterior of the whole root is the piliferous layer, 

 which bears numerous unicellular root-hairs. We see 

 that, except for minute details, such a root resembles a 

 young root of the Wallflower (see Part I. p. 73), but in 

 the case of the Ferns there is no secondary growth of 

 thickness. When we come to consider the develop- 

 ment, we shall find considerable differences from any of 

 the previous types. 



4. The Growing-Points 

 a. The Stem 



With rare exceptions, the development of both stem 

 and root in Ferns can be referred to a single apical 

 cell, from the divisions of which all tissues and organs 

 arise. This important cell can be easily distinguished 

 from its neighbours, which are derived from it, by its 

 larger size and characteristic form. In the Male Fern 

 and most other members of the class, the apical cell 

 of the stem has the form of an inverted three-sided 

 pyramid or tetrahedron, with its curved base directed 

 outwards (see Fig. 26, which is taken from a simpler 

 Fern-stem, but illustrates the essential features). 



In longitudinal section, therefore, the cell appears 

 triangular ; its three sides are in contact with the 

 adjacent tissue, while the curved base is free and faces 

 upwards (assuming the stem to be erect). The apical 



