SECT. vin. FRUCTIFICATION. 341 



When a fern acquires a considerable stem, as in Tree 

 Ferns, it consists of a central or medullary part, con- 

 sisting of cellular tissue, a.nd an external or cortical 

 portion, formed of the consolidated bases of the fronds, 

 in which may be seen, on cutting a section of a trunk, 

 an irregular zone, formed of fibro- vascular bundles, 

 scalariform ducts, and woody fibre. Prolongations from 

 this zone pass into the leaf- 

 stalks, and thence into the 

 midrib of the leaf, whence 

 they spread into its lateral 

 branches, and ultimately ap- 

 pear in the leafy parts in the 

 form of veins. It is the ar- 

 rangement of these bundles 

 of coloured woody tissue in 

 the cellular tissue of the leaf- 

 stalks of the herbaceous ferns, 



Which give rise tO those peCU- Fig. M. Section of footstalk of Fern 



liar figures on their transverse 



sections, such as a star, the letter T, and the heraldic 

 spread eagle, from which latter the common Brake Fern 

 (Pteris) takes its botanical specific name of aquilina. 

 Fig. 54 shows an oblique section of the footstalk of a 

 fern leaf with its bundles of scalariform ducts, as deter- 

 mined by Dr. Carpenter's microscopic observations. 



The fructification of the ferns is arranged with the 

 most perfect symmetry, usually on the under surface of 

 the leaf, but sometimes at the margin, and assuming a 

 great variety of forms and positions. It consists of 

 sori, that is, groups of nearly globular spore cases or 

 sporangia, sometimes sessile on the frond, sometimes 

 with a footstalk. They are always situated on a vein 

 or its branches, or at the extremity of the veins on the 

 margin of the frond. In fact the sporangia originate in 

 the cellular tissue immediately in contact with a vein, 



