160 CEYPTOGAMOUS OR FLOWERLESS PLANTS. [SECTION 17. 



closely resembles a small Liverwort. This is named a Pkothallus (Fig. 

 509) : from some point of this a bud appears to originate, which produces 

 the first fern-leaf, soon followed by a second and third, and so the stem 

 and leaves of the plant are set up. 



490. Investigation of this prothallus under the microscope resulted in 

 the discovaiy of a wholly unsuspected kind of fertilization, taking place at 



this germinating stage of the plant. On the under side of the prothallus 

 two kinds of organs appear (Fig. 510). One may be likened to an open 

 and depressed ovule, with a single cell at bottom answering to nucleus ; 

 the other, to an anther; but instead of pollen, it discharges corkscrew- 

 shaped microscopic filaments, which bear some cilia of extreme tenuity, by 

 the rapid vibration of which the filaments move freely over a wet surface. 

 These filaments travel over the surface of the prothallus, and even to other 

 prothalli (for there kre natural hybrid Ferns), reach and enter the ovule- 



FiG. 511. Lyoopodium Carolinianum, of nearly natural size. 512. Inside view 

 of one of the bracts and spore-oase, magnified. 



Fig. 513. Open 4-valved spore-case of a Selaginella, and its four large spores 

 (raacrospores), magnified. 514. Macrospores of another Selaginella. 515. Same 

 separated. 



FiQ. 516. Plant of Isoetes. 517. Base of a leaf and contained sporocarp filled 

 •with microspores cut across, magnified. 518. Same divided lengthwise, equally 

 magnified ; some microspores seen at the left. 519. Section of a spore-case contain- 

 ing macrospores, equally magnified i at the right three macrospores more magnified. 



