THE PTERIDOPHYTA : FILICALES, THE FERNS 515 



out by axillary buds, developed at the nodes, of which the majority remain 

 dormant. Axillar\' buds are rare among Ferns, and it has been suggested 

 that in this instance the axillary bud is really the weaker or suppressed branch 

 of a dichotomy of the main stem, and that the leaf is really the first leaf of 

 this branch. On the other hand, a comparison with Ferns which show equal 

 dichotomy leads to a different interpretation. In dichotomies with equal 

 branching there is always present a leaf which stands at the point of forking 

 and is called the angular leaf. Such leaves are characteristic of Ferns and 

 do not occur in Lycopodiales. When the two branches are somewhat unequal, 

 the angular leaf is often shifted on to the weaker branch, as in Pteridium. 

 This may be the case in HymenophyUum, and the leaf may be really an angular 

 leaf between the main branch and the side branch. 



The Filicales show no such fixed relationship between branch and leaf 

 as obtains in the Angiosperms. The evolutionary trend has been from 

 dichotomy towards true axillary branching and in the Filicales the arrange- 

 ment is still fluid. Lateral branches may arise on either side of the leaf- 

 insertion, or in the leaf axil, or below the leaf, or finally, the branch may be 

 shifted on to the leaf itself or the leaf on to the branch. There is thus great 

 diversit}-, illustrating the course of evolution towards the fixed type of 

 axillary branching, which was attained at an early stage in the Equisetales 

 but not in the Filicales or their immediate relations. The angular leaf at a 

 dichotomy is, however, clearly the prototype of the subtending leaf of an 

 axillary branch, and the whole tendency of evolution has been to fix the 

 branching system in relation to particular leaves. 



Anatomy of the Stem 



There is a broad cortex which shows a thin-walled outer zone and a 

 thick-walled, dark-coloured, inner zone. The single, central stele is delimited 

 by a typical endodermis, within which is a pericycle of one to several layers 

 of thin-walled cells (Fig. 510). The xylem forms a central core, surrounded 

 by a continuous phloem. The xylem is solid, with central protoxylem, and 

 is therefore protostelic. \ small number of parenchyma cells are scattered 

 among the tracheids towards the centre of the stele and may, in some large 

 species, be aggregated into a ring which separates the protoxylem from the 

 metaxylem. 



The protoxylem and some tracheids of the metaxylem form a simple 

 collateral strand in the leaf trace, which naturally leaves no gap in the stele. 

 In the rachis this strand widens into a more or less curved arc (Fig. 511). 



Anatomy of the Leaf 



The lamina is formed of one layer of cells only, except in a few species, 

 and has the delicate translucent texture of a Fern prothallus (Fig. 512). It is 

 sensitive to bright light and to low atmospheric humidity and prefers moisture 

 and deep shade. The venation of the leaves is branched and open, without 



