GROUP III. PTERIDOPHYTA : IiYCOPODINJl. 425 



on the discoid central region of the upper surface. Whilst in the foregoing 

 cases the prothallium is very small and simple, in L. Phlegmaria and sprue 

 other 'species (L. carinatum and Hippuris) it is considerably larger and more 

 complex. It consists here of a cylindrical, monopodially-branched body, with 

 apical growing-points similar in structure to those of the sporophyte. The 

 superficial layer of cells, representing an epidermis, gives rise to a number of 

 root-hairs. The sexual organs are developed on special branches, yametophores, 

 though antheridia sometimes occur on the vegetative branches ; the gameto- 

 phores are shorter and thicker than the vegetative branches, sometimes even 

 tuberous, and bear the sexual organs on the upper surface surrounded by stout 

 multicellular hairs, paraphyses. 



The male organs, antheridia, are sunk in the tissue of the prothallium : they 

 resemble those of the Eusporangiate Filiciuffi. Their development precedes that 

 of the female organs. 



The male cells, spermatozoids, are oval in shape, and have two cilia : a vesicle 

 is extruded from the mother-cell together with the spermatozoid, as in the Filicinae. 



The female organs, archegonia, have short necks which project but little 

 above the surface of the prothallium. They have the structure usual among 

 Pteridophyta : in L. Phlegmaria, there are generally several neck-canal-cells. 



The female cell, oosphere, requires no special description. 



In consequence of its position and of its mode of development, the embryo is 

 forced downwards into the tissue of the fleshy prothallium, being anchored, as 

 it were, at one end by the suspensor. As it grows it destroys the cells of the 

 prothallium with which it comes into contact, and absorbs the nutritive sub- 

 stances stored in these cells by means of the so-called foot, the superficial cells 

 of which grow out into short papilla?. In its further growth the embryo becomes 

 more and more curved until it regains the surface of the prothallium and pro- 

 jects. In L. Phlegmaria the embryo remains for some time enclosed in a sac, 

 the calyptra, formed by active growth of the prothallial tissue. 



The life of the prothallium is short and closes, in most cases, with the 

 development of an embryo from the oospore, but in L. Phlegmaria it seems to 

 persist from one season to another. In the latter species the prothallia are 

 multiplied vegetatively by the isolation of branches, as a,\*o by small multicellular 

 bulbils. 



A remarkable biological feature is the constant association of a Fungus 

 (probably a Pythium) with the prothallia, as also with the embryo, of L. cernnunt 

 and inundatuni. 



Order 2. Psilotaceae. This order consists of the two genera Psilotum and 

 Tmesipteris ; of these the former is widely distributed in the tropics ; the 

 latter is coutined to Australia, New Zealand, and Polynesia, and lives epi- 

 phytically, aud perhaps parasitically, on the trunks of Tree-Ferns. 



THE SPOROPHYTE. The most striking feature in the morphology of these plants 

 is the total absence of roots, the functions of these organs being performed by 

 specially adapted stem-brauches bearing minute scale-leaves, and covered with 

 root-hairs. 



The stem. In Psilotum the subterranean shoots have unlimited apical growth : 

 they are much branched, apparently dichotomously, but it seems probable that 

 the branching is really lateral. Tue subaerial shoots generally arise as lateral 



