THE PTERIDOPHYTA : FILICALES, THE FERNS 503 



The diploid generation is called the sporophyte, while the monoploid part 

 of the life-cycle is called the gametophyte. Thus the alternation of two 

 morphologically different generations corresponds to two cytologically 

 different phases (Fig. 494). 



Sporangium 



Fern plant- 

 Embryo 



'Antherozoids 

 / 



Archegonium 



itheridium 



.Prothallus. 



Fig. 494. — Life-cycle of Dryopteris filix-mas. 



Pteridium aquilinum (The Bracken Fern) 



The common Bracken Fern, Pteridium aquilinum, occurs very commonly 

 in this country on open heath land and in sandv woods (Fig. 495). In its 

 life-history, and to some extent in its structure, it resembles the Male Fern, 

 but there are certain important differences. 



The plant consists of a creeping underground stem or rhizome, from 

 which are given off vertically growing leaves. Each leaf consists of a very 

 long stem-like petiole and a large, compound lamina, which is triangular 

 in outline. Pinnae are few^ and are stalked ; each bears stalked pinnules, 

 and these are again subdivided. We realise, therefore, that the only part 

 of the Bracken Fern which we normally see above ground is the leaves, and 

 the structure which is frequently erroneously regarded as the stem is really 

 the petiole of the leaf. 



The true stem is the blackish-coloured rhizome, which grows very vigor- 

 ously underground and is the plant's chief means of spreading. It grows 

 by means of a single wedge-shaped, i.e. two-sided, apical cell (Fig. 496), and 

 branches dichotomously, that is, by division of the apical cell, into two equal 

 parts. The two limbs do not, however, develop equally (Fig. 497). One 

 branch usually remains very short for some time and bears a leaf so close 

 to its base that the apex looks like an adventitious bud on the base of the leaf 

 petiole. 



This leaf is, in fact, an angular leaf, that is to say, it originates 



