132 



MANUAL OF BOTANY 



adventitious root, which grows out through the fundamental 

 tissue m the way akeady described {fig. 888). Occasionally, as 

 in Osmunda, there may be a group of initial cells at the apex, 

 instead of the usual pyramidal one. 



The root is monostelic, the stele containing sometimes two, 

 sometimes three bundles ; the pericycle may consist of a single 

 layer of cells or of many, and sometimes it is irregular, being one 

 layer of cells thick in part of its com:se and more than one layer 

 thick in the remamder. 



When the roots branch the new root springs from the old 

 one in the same way as the latter did frcm the stem. 



The foliage leaves arise each from a single superficial cell of 



the growing point of 

 Fig. 888. the stem. They grow 



in most cases by 



means of a two-sided 

 apical cell. In verna- 

 tion they are strongly 

 curled up owing to the 

 greater growth of the 

 under surface. Later 

 the region of growth 

 changes to the upper 

 surface and the leaves 

 expand. In structm-e 

 they resemble the 

 leaves of Angiosperms, 

 but the differentiation 

 of the mesophyll into 

 palisade and spongy 

 parenchyma is not so well marked. The epidermal cells as a 

 rule contain chloroplastids. The vascular cylinder is generally 

 monostelic in the petiole, but becomes schizostelic in the leaf 

 blade. The stele joins one of the steles of the stem as already 

 described. 



In some Ferns the sporangia are borne upon leaves that 

 cannot be distinguished in appearance from ordinary foliage 

 leaves. In others, as in Osminida, Aneimia, &c., there are 

 definite, specialised sporophyUs. These show various shapes, the 

 pinnae generally being narrower than those of the foliage leaves. 

 In the most specialised, the upper or lower branches of the 

 phyUopodium are scarcely at all winged, the edges being covered 

 by clusters of sporangia. 



Fig. 888. Longitudinal section through root of 

 Pteris hastata, showing apical region, v. Apical 

 cell, from which are developed the tissue of the 

 substance of the root, o, c, and the root- cap, 

 it, I, 771, n. After Sachs. 



