The structure & development of the mosses and ferns (Archegoniatae) . week, depending upon the temperature. Theexospore is ruptured irregularly near one end, and through this ashort colourless papilla protrudes and is shut off by a transversewall (Fig. 156, B). This papilla contains little or no chloro-phyll and rapidly lengthens to form the first root-hair, whichundergoes no further divisions. The large green cell aloneproduces the prothallium. The divisions in the prothallialcell vary somewhat, but in the great majority of cases a seriesof transverse walls is first formed, and the young prot
The structure & development of the mosses and ferns (Archegoniatae) . week, depending upon the temperature. Theexospore is ruptured irregularly near one end, and through this ashort colourless papilla protrudes and is shut off by a transversewall (Fig. 156, B). This papilla contains little or no chloro-phyll and rapidly lengthens to form the first root-hair, whichundergoes no further divisions. The large green cell aloneproduces the prothallium. The divisions in the prothallialcell vary somewhat, but in the great majority of cases a seriesof transverse walls is first formed, and the young prothallium(Fig. 156, C) has the form of a short filament. Sooner orlater, in normally-developed prothallia, the terminal cell of therow becomes divided^ by a longitudinal wall, which may bestraight, but more frequently is oblique and followed by anothersimilar wall in the larger of the two cells, meeting it so as to FILICINE^ LEPTOSPORANGIAT^ 309 include a triangular cell, which is the two-sided apical cell ofthe next phase of the prothalliums growth. The divisions up. Fig. 156.—Onoclea stnithioptcris (Hoffm). A, B, Germinating spores with the perinium removed,X 300 ; C, young prothallium, x 100 ; D, E, older prothallia with two-sided apical cell {x), X 300 ; F,small female prothallium seen from below, X25; G, very young prothallium with the two outerspore-coats, X 300 ; r, primary rhizoid ; ar, archegonia ; p, perinium ; ex, exospore. to this point correspond exactly with those of Aneura orMetzgeria, and are also much the same as in Marattia, except 3IO MOSSES AND FERNS chap. that here the prothallium only in very rare cases assumes theform of a cell mass at first. By the regularly alternating segments of the apical cellthe young prothallium soon assumes a spatulate form, whichbecomes heart-shaped by the rapid growth of the outer cells ofthe young segments, which grow out beyond the apical or later the single apical cell is replaced by two ormore initials formed fro
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