208 THIRD GROUP. VASCULAR CRYPTOGAMS. 



Either female sexual organs occur on them in abundance, but are infertile (Todea\ or 

 they are very rare (Pteris cretica\ or they are not found at all 1 (Aspidium Filix-mas 

 var. cristatum). We have already pointed out a similar advance from step to step in 

 the apogamy of the Saprolegnieae, and we shall have to notice analogous phenomena 

 in some of the Phanerogams (see too Isoetes) 2 . 



The full-grown Fern, to the consideration of which we now return, is in many of 

 the Hymenophyllaceae a small and delicate plant not greatly exceeding the dimensions 

 of the large Muscineae. In the other divisions fully developed specimens are usually 

 handsome plants, and many species from the tropics and the southern hemisphere 

 have the habit of a palm, and are known as Tree-ferns. The stem either creeps on 

 or in the ground (Polypodmm^ Pteris aquilina), or climbs on rocks or the stems of 

 other plants, or ascends obliquely, as in Aspidium Filix-mas and many others, or 

 rises erect and columnar as in the Tree-ferns. They have usually a very abundant 

 formation of roots ; the stem of many Tree-ferns is often quite covered with a thick 

 mass of downward growing roots. The roots arise on the stem in acropetal 

 succession, sometimes close behind the growing apex, as in Pleris aquilina. If the 

 internodes remain very short and the stem is covered with leaf-scars, the roots often 

 spring from the leaf-stalks, as in Aspidium Filix-mas. In many Hymenophyllaceae 

 without true roots branches from the stem assume the form of roots, as was mentioned 

 above. The leaves in the creeping and climbing forms, and in many of those which 

 grow free and erect, are separated by evident or even long internodes; in thick 

 stems, both ascending and vertical, the internodes are usually not developed, and the 

 leaves are placed so close together that no bit of the surface of the stem, or a 

 very inconsiderable part of it, remains free and uncovered by them 3 . Many stems 

 show in the arrangement of their parts a distinction between a ventral side which is 

 turned to the substratum and a dorsal side ; they are in a word dorsiventral. Thus in 

 many Hymenophyllaceae the leaves are on the dorsal side of the stem ; the same 

 thing may be seen but less distinctly in some species of Polypodium, P. vulgare for 

 example and P. aureum, where two rows of leaves stand near each other on the dorsal 

 side, while in Lygodium there is only a single dorsal row of leaves present from the first 4 . 



1 See the description of the phenomena connected with apogamy in the Saprolegnieae and As- 

 comycetes in preceding portions of this work. 



2 [The converse condition, the direct passage from the Fern (sporophyte) to the prothallium 

 (oophyte), aptly termed by Bower apospory, has been observed in Athyrium Filix-fem 'na var. 

 clarissima by Druery and in Polystichum angulare wt.pulcherrimum by Wollaston. From Bower's 

 description we learn that in the former plant sporangia attached to the pinnae and arrested in their 

 normal development exhibit active vegetative growth of the stalk and of the superficial cells of 

 the capsule resulting in the formation of a flattened parenchymatous structure with chlorophyll- 

 cells and growth by wedge-shaped cells at one or more points of the margin, in fact prothallia. 

 Sexual organs are developed upon the prothallia. In the second plant prothallia bearing sexual 

 organs are formed by simple vegetative outgrowths of the tip of the pinnae without any connection 

 with sori or sporangia. The degrees of apospory observed in these cases may be compared with 

 those of apogamy in the text, and it may be noted in connection with the explanation of the origin 

 of apogamy that the cases of apospory known occur also in garden varieties. See Druery in Journ. 

 Linn. Soc. XXI, 1885 ; Bower in same vol. ; Thiselton Dyer in Nature, No. 791, and Wollaston in 

 Card. Chron. XXIV, 1885.] 



3 Brongniart concluded from the changes in shape and size of the older leaves that the stems 

 of Tree-ferns grow in length and perhaps in thickness also after the leaves have fallen. 



4 See also what is said below with regard to the branching. 



