230 



THE DICTIONARY OF GARDENING, 



Prothallus, or Prothallium continued. 

 sub-divided by cell walls. The result is that a structure 

 is formed in the shape of a flask with a long, narrow 

 neck. The hollow of the flask is occupied by a large 



FIG. 295. IMMATURE ARCHEGONIUM, much magnified, n, Neck- 

 cells; c, Canal, still closed above, and filled by the Canal- 

 cell ; o, Oosphere ; e, e, Epidermis of Prothallus. 



cell, the oosphere, rich in protoplasm (see Fig. 295). The 

 tube of the neck is at first filled with a narrow cell, 

 the canal-cell, the cell wall of which becomes muci- 

 laginous, swells, and is expelled from the outer opening 

 of the tube, leaving a passage for the antherozoid down 

 tie tube to the oosphere, when the latter is ripe to be 



FIG. 296. LONGITUDINAL SECTION OF MATURE ARCHEGONTUM OF 

 FERN, much magnified n, Neck -cells ; c, Opening of Canal 

 down neck ; o, Oospbere ; e, e, Epidermis of Prothallus. 



acted on by it (see Fig. 296). The antherozoids are caught 

 in the mucilage while moving over the moist Prothallus ; 

 they wriggle down the tube, reach the oosphere, and 

 fertilise it. The latter very soon begins to grow ; and the 

 final result is the development of the oospore into the 

 leafy plant or Fern. It may be mentioned that the 



FIG. 297. DIAGRAMMATIC SKETCH OP CONNECTION OF YOUNG 

 FERN WITH PROTHALLUS-J>, Prothallus; rh. Root-hairs of 

 Prothallus ; f Foot of Young Fern, imbedded in hoflow of 

 enlarged Archegonium, a ; fr, Very Young Frond of Fern 

 r. Root of Fern. 



oospore, at a very early period, divides into eight cells, 

 in two layers. Of th,ese cells, four lie next the base, and 

 four next the front margin of the Prothallus. Of the 

 latter, the two farthest from the neck of the archegoninm 

 give origin to the first leaf or frond ; one, near the neck, 

 to the growing point of the stem ; and the fourth to hairs. 

 Of the other four cells, one, opposite to the stem, de- 

 velops into the root, one ultimately disappears, and the 

 other two form the " foot," a structure that remains sunk 

 in the archegonium, which has grown so as still to sur- 

 round the foot (see Figs. 297 and 298). By means of this 



Prothallus, or Prothallium continued, 

 organ, the young plant absorbs nourishment from the 

 Prothallus, which, for a time, increases in size, but is 

 gradually used up, and withers away, and afterwards 

 the young Fern is able to nourish itself by its own 

 roots and loaves. 



FIG. 298. YOUNG FERN GROWING FROM PROTHALI.US, slightly 

 enlarged p, Lower Surface of Prothallus ; rh, Root-hairs of 

 Prothallus; fr, Younj; Frond of Fern ; r, Root of Fern. 



Two departures from this mode of reproduction have 

 been detected in Ferns within recent years. The ono 

 of these, called " apogamy " (from apo, afar, and gamos, 

 marriage) by Professor de Bary, was detected, in Pteris 

 cretica, by Professor Farlow, and is now known to occur 

 in a few other Ferns, including Nephrodium Filix-mas 

 cristatum. In this process, the young Fern is produced 

 as a bud from certain parts of the Prothallns, without 

 the formation of sexual organs. The sexual process in 

 this case is abolished, as the name indicates. 



In 1884, Mr. Druery stated, in the Linnaean Society, 

 the discovery that, in certain Ferns, the Prothalli are 

 produced as outgrowths from the pinnules of the Fern 

 fronds, and not from the spores. This process has been 

 called "apospory" (from apo, afar, and spora, a sj ore or 

 seed). It has been investigated and described by Pro- 

 fessor Bower in examples supplied by Mr. Druery, of 

 Athyrium Filix-fcemina clarissima, and Polystichum an- 

 gulare pulcherrimum. In this departure, the produc- 

 tion of spores is suppressed ; the Prothalli in the former 

 being modified sporangia, while, in the latter Fern, no 

 trace of the sporangium even can be detected. The 

 sexual reproduction is not affected, and the leafy Feres 

 are developed from the Prothalli in the usual way. 



Though of very great scientific interest, the develop- 

 ment of Prothalli, and of the sexual organs on them, is 

 of less practical importance to gardeners in the other 

 groups of Vascular Cryptogams than it is in the true 

 Ferns ; but an outline of the chief points of difference 

 in these groups may be given. In the small group 

 Ophioglossece, represented in the British Flora by the 

 Moonwort and Adder's Tongue Ferns, the Prothallus is 

 formed underground, is destitute of chlorophyll, and 

 is usually formed of a mass of cells. It produces sexual 

 organs, which resemble those of Ferns in the main. The 

 Equisetinew, or Horsetails, resemble Ferns in the Pro- 

 thalli being green, flattened layers of cells, growing on 

 damp surfaces; but they become branched into long, 

 narrow lobes, and may reach in. in length. They are 

 dioecious, i.e., each produces only antheridia or arche- 

 gonia. The former are produced near the tips of the 

 lobes of the male Prothalli; the latter usually in the 

 clefts between the fleshy lobes of the female Prothalli. 

 The development of the sexual org-ans, and of the 

 " leafy plant " (if an Equisetum deserves this designa- 

 tion), calls for no special comment here, as it agrees in 

 the main points with that in Ferns. 



The Club-mosses fall into two groups, of which one, the 

 Lycopodiece, much require to have their development 

 worked out. So far as is known, their Prothalli are 

 irregnlarly-lobed masses of cellular tissue, and bear both 



