5IO 



A TEXTBOOK OF THEORETICAL BOTANY 



zone and separate the two phloems into four, giving a superficial resemblance 

 to a tetrarch instead of a diarch structure (Fig. 505). 



There is a remarkable anatomical similarity among all Fern roots, even 

 in fossil Ferns from the Carboniferous, however widely the stem structures 

 may differ. This is sometimes expressed by the saying that the root is a 

 " conservative organ." 



The Sporangium 



The sporangia of the Bracken Fern form a continuous line close beneath 

 the margin of the leaf instead of being distributed in separate sori (Fig. 506). 

 The placenta actually originates from the leaf margin itself, but it is concealed 

 by an indusium which is folded over it and gives the appearance of being an 

 infolded leaf margin. A second indusium is also formed on the inner side 

 of the placenta, so that the sporangia are enclosed between them, a peculiarity 

 of this Fern. Both these indusia are outgrowths from the leaf surface. 



The development of the sporangia is essentially similar to that in the 

 Male Fern. They arise without any regular succession on the placenta, 

 and each possesses an annulus and stomium similar to that of Dryoptens 

 (Fig. 507). Within the sporangium numerous spores become differentiated 

 (Fig. 508), which are liberated by the bursting of the sporangium at the 

 stomium under the influence of the contracting annulus. 



The Gametophyte 



The spores on being shed germinate to produce a prothallus, which is 

 similar in appearance to that of Dryoptens. On this prothallus, antheridia and 

 archegonia develop which give rise respectively to antherozoids and an 

 oosphere, and as a result of the conjugation of these gametes an embryo is 

 formed which develops into a fresh Bracken plant. In all these stages there 

 is no important distinction between Dryoptens and Pteridium, and it will not, 

 therefore, be necessary for us to consider them in detail. 



Leptosporangiatae : Hymenophyllaceae 



The Hymenophyllaceae or " Filmy Ferns " are a small group of Ferns 

 with very delicate leaves which grow in very damp and shady places, usually 

 in forests. In tropical rain forests many species are epiphytic. 



The slender, protostelic stems are creeping, and usually have adventitious 

 roots, though in some forms the rhizome-branches function as roots and are 

 provided with root hairs. The leaves are borne in two row^s and the lamina 

 is only one cell thick and translucent. The sori are marginal, the placenta 

 being formed by the prolongation of the end of a vein beyond the edge of 

 the leaf. The sorus is enclosed by a cup-like indusium, which may be two- 

 lipped or divided into two valves. The sporangia are sessile, with an oblique 

 annulus, and develop in basipetal succession on the placenta {Gradatae of 

 Bower). The prothallus is filamentous or strap-shaped. 



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