FILICINE^ 435 



parent root. The cortex of the parent root is simply broken through, no root-sheath 

 being formed. 



The Trichomes of Ferns assume a great variety of forms. True root-hairs, 

 simple unarticulated tubes, arise, not only on the roots themselves, but also on 

 underground stems and on the bases of leaf-stalks (as in 

 Pieris aquilina and Hymenophyllaceae). On aerial creeping 

 stems and on the leaf-stalks the numerous usually brownish 

 or dark-brown flat multicellular hairs, the Palece or Ramenia, 

 occur, soon becoming dry, often entirely enclosing the buds, 

 and attaining a length of from i to 6 cm. (as in Poly- 

 podium, Cihoiium, &c.). Long strong bristles are sometimes 

 found on the lamina (in Acrosiichum crinitum), and very often 

 fine, delicate, articulated hairs. They are formed from single 

 superficial cells at the growdng-point. 



The Sporangia of Ferns are small rounded capsules, which 

 are borne on long stalks in the Polypodiacese and Cyatheaceae, fig. sos.-under-side 



1 1 • 1 M • 1 -r-> ' rT-<i ^^ r ^ °^ ^ lacinia of a leaf of 



but which are sessile m other Terns. 1 he wall of the sporan- Aspidium Futx-mts. 



. ' r 1 .11 /• 11 . ^''^'^ eiglit indusia i 



gium, when mature, consists of but a single layer of cells. A (X2). 



ring of cells belonging to the wall of the capsule and running 



across it transversely or obliquely or lengthwise is generally developed in a peculiar 



manner, and is then termed the Annulus. By its contraction when dried up the 



capsule bursts at right angles to jthe plane of the annulus. Sometimes, instead 



of the annulas, a terminal or lateral group of the cells of the wall of the capsule 



is developed in a similar manner. 



The sporangia are generally combined into groups, each group being termed a 

 Sorus; the sorus contains either a small definite number or a large indefinite 

 number of sporangia, and among them also very commonly some slender articu- 

 lated hairs, the Paraphyses. The whole sorus is very generally covered by an 

 excrescence of the epidermis, the true Indusium^ : in other cases the indusium 

 consists of an outgrowth of the tissue of the leaf itself, and is then composed of 

 several layers, and even has stomata ; or the covering of the sorus is simply the. 

 result of the margin of the leaf being recurved or rolled over it : in these cases the 

 indusium is said to ht false. In Lygodium each separate sporangium is covered by a 

 pocket-shaped growth of the tissue of the leaf like a bract. Sori are not usually 

 formed upon all the leaves of the mature plant; sometimes groups of fertile and 

 sterile leaves alternate in regular succession, as in Struthiopteris ger??iattica. In some 

 cases the sori are uniformly distributed over the whole of the lamina, in others they 

 are connected with definite portions of it. The fertile leaves may be in other 

 respects hke the sterile ones, or they may be strikingly diff"crent from them ; and 

 this difference is not unfrequently occasioned by the partial or entire failure of deve- 

 lopment of the mesophyll between and near the fertile veins ; the fertile leaf, or the 

 fertile part of the leaf, then appears like a spike or panicle furnished with sporangia 

 {e, g. Osmunda, Anemia). The sporangia generally arise from the epidermis of the 



^ [On the development of the indusium, see Prantl,' Die Hymenophyllaceen, 1875, and die 

 Schizseaceen, 1881 ; Burck, Over de ontwikkelingsgeschidenis van het indusium der Varens, 

 Haarlem 1874.] 



F f 2 



