THE PTERIDOPHYTA : FILICALES, THE FERNS 483 



The leaves, which are usually spoken of as fronds, may be from i to 

 3 ft. in length and are much divided. A divided leaf of this kind is termed 

 a compound leaf, and is composed of a number of separate leaflets, which 

 in the Male Fern are called pinnae. These are again subdivided into 

 pinnules. The pinnae are borne on a main stalk or rachis, w^hich is a 

 continuation of the petiole. In the young state both the petiole and rachis 

 are clothed with dry, brown scales or ramenta. 



The leaf takes two years to mature, and in their first year both the petiole 

 and the rachis are coiled up in the form of a helix, and each leaflet is similarly 

 coiled. Such a method of folding is spoken of as circinnate, from the Latin 

 circinmis, a bishop's crozier (Fig. 471). The mature leaf only lasts one 

 season and a crop of ten to twenty fresh leaves is produced each year, in a 

 spiral succession continuous with those of previous years. The rudiments 

 of next year's leav'es form a close group over the stem apex. 



The roots are of two kinds ; firstly the single main root, which is produced 

 by the embryo and dies away very early, and secondly roots which are termed 

 adventitious, because they are produced secondarily from the tissues of 

 the stem. Generally three such roots are produced at the base of each petiole. 

 These roots are very slender and extensively branched, so that an old stem 

 becomes almost completely covered by a mass of adventitious roots. 



Anatomy of the Stem 



In the simpler members of the Pteridophyta the stem consists of a single 

 central stele surrounded by a cortex, but in many Ferns the structure becomes 

 more complicated by the breaking up of this stele into a ring of steles which 

 form a network. Each piece of this network is termed a meristele, and the 

 whole structure is said to be dictyostelic (see p. 561). 



Fig. 472 is a photograph of the stelar structure of a Fern stem from w^hich 

 all the parenchymatous tissue has been removed, leaving only the vascular 

 skeleton. The meristeles form a tubular network with large diamond-shaped 

 meshes, and each mesh corresponds to the base of a leaf. These are the 

 leaf gaps. From the lower side of each mesh a number of slender meristeles 

 emerge. These are the leaf traces which in the intact stem traverse the 

 cortex and enter the petioles. Since the leaf bases of the Male Fern entirely 

 cover the surface of the stem, any transverse section cut across the stem must 

 necessarily be surrounded by the bases of leaves, whose traces are cut 

 obliquely (Fig. 473). 



Each meristele consists of a central core of xylem surrounded by phloem. 

 The xylem is composed of scalariform tracheids and xylem parenchyma. 

 The tracheids are large and polygonal in transverse section. There are no 

 vessels in Dryopteris. Near the centre in small circular meristeles, or at the 

 foci in larger elliptical ones, are one or two groups of much smaller tracheids 

 which bear spiral or reticulate thickenings. These are the protoxylem 

 elements, that is to say, the first xylem cells formed from the apical meristem. 

 They dift'er, as usual, in their thickening from the mature xylem or 

 metaxylem. 



