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 sclerenchyme of 

 the fundamental tis- 

 sue the sclerosis 

 may take place in 



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

 leiorftizwn Wall, p, fundamental tissue ; J, scterenchymatous 

 .sheath ; b, phloem ; A, xylem ( x 200). (After Luerssen.) 



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 Polypodiacese and Osmundacese 

 the entire cortex assumes eventually 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 solidity 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 dosed or destitute of cambium ; in the stem, 

 except in Osmunda, 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 



