FILICES 



75 



and this is true not only of the erect columnar stem of tree-ferns, but 

 also of the creeping or erect stem of smaller species. Axillary branch- 

 ing is very rare, if it ever actually occurs ; the terminal branching is 

 always dichotomous, never sympodial. The fundamental tissue of the 

 stem and leaf-stalk consists, in many species, entirely of thin-walled 

 parenchyme. In others, and especially in tree-ferns, portions of it 

 undergo a change in the great thickening and brown colouring of the 

 cell-walls, the cells becoming at the same time prosenchymatous. In 



this sderenchyine of 

 the fundamental tis- 

 sue the sclerosis 

 may take place in 



Fig. 53. — Transverse section of 'vascular' bundle of Polypoducm 

 leiorhizum Wall, p, fundamental tissue ; s, sclerenchj-matous 

 sheath ; b, phloem ; hy xji-lem ( x 200). (After Luerssen.) 



pr 



Fig. 54. — Transverse 

 section of stem of 

 Pteris aquilina L. 

 r, epiderm ; /, fun- 

 damental tissue ; pr, 

 sclerenchymatous 

 sheath ; ig, vascular 

 bundle; ag, outer 

 network (somewhat 

 magnified). 



individual isolated 

 cells ; more often 

 the cells so affected 

 are united into con- 

 spicuous bands or sheaths. In many Polypodiaceae and Osmundaceae 

 the entire cortex assumes eventuallv a dark colour. In the common 

 brake (Pteris aquilina, L.) two thick sclerenchymatous bands of this 

 description lie between the inner and outer ' vascular ' bundles, 

 while another continuous layer immediately underlies the epiderm. 

 The firmness and sohdity of the stem of tree-ferns are mainly due to 

 strongly developed sclerenchymatous cylinders which form complete 

 sheaths surrounding the ' vascular ' bundles. The ' vascular ' bundles 

 themselves are always closed or destitute of cambium ; in the stem, 

 except in Osnmnda, and usually in the leaves, they are concentric^ consist- 

 ing of a central xylem-portion entirely enveloped in a layer of phloem ; 

 in the stem and leaves of Osmunda, and in the leaves of some other 

 ferns, they are collateral^ the xylem and phloem portions lying side by 



