3 i8 



FOURTH GROUP. SEED-PLANTS. 



the tissue of the pro thallium^ which then produces archegonia beneath the apex of the 

 macrospore. The presence of a ventral canal-cell seems to be still doubtful * ; the 

 number of cells in the canal of the neck is limited to two, and these often form lobe- 

 like outgrowths. A cavity is formed beneath the mieropyle by absorption of a portion 

 of the tissue of the macrosporangium known as the pollen-chamber (the ' chambre 

 pollinique ' of Brongniart), and in which the pollen or microspore lies after it has 

 passed through the mieropyle. The embryo in many of the Cycadeae as in many 

 Coniferae is not formed till after the dispersion of the seed, and the mode of formation 

 is in both cases imperfectly known ; in each archegonium is produced a suspensor as 

 in Selaginella. As in many heterosporous Vascular Cryptogams, such as Salvinia and 



P/lu/aria, the prothallium of the Cycadeae 

 has the power of independent further develop- 

 ment, if no fertilisation takes place ; it bursts 

 through the tissue that surrounds it and grows 

 green in the light. Only one of the rudi- 

 mentary embryos developes, but the sus- 

 pensors of the others may still be recognised 

 as coils of long threads in the ripe seeds. 

 Pollination appears to be effected by the 

 wind ; the mieropyle secretes a fluid in which 

 the pollen-grains are caught, and are then 

 drawn down into the cavity. beneath the mi- 

 eropyle, from whence they no doubt send 

 their tubes as far as the archegonia. There 

 is no fixed number of cotyledons ; Cerato- 

 zamia has only one, Cycas and Zamia two 

 which lie with their inner faces flat on one 

 another and cohere towards their apex ; the 

 tendency to branch which is shown by the 

 later foliage-leaves appears sometimes in the 

 larger cotyledons by the formation of a rudi- 

 mentary lamina with indication of pinnae, as 

 in Zamia (Fig. 249 J9'). The seed germi- 

 nates in damp ground but not till after the 

 lapse of some time ; the seed-coat bursts at 

 the posterior extremity and releases the 



primary root which at first grows vigorously downwards, and afterwards sometimes 

 enlarges like the root of the turnip, or developes a system of thick filamentous 

 roots. According to Fig. 249 C taken from Schacht and the more recent state- 

 ments of Reinke the branching of the primary root is monopodial, but Miquel speaks 

 repeatedly of dichotomous divisions in the more slender roots of older plants of Cycas 



FIG. 249. B, ', C germination of Zamia spiralis reduced. 

 B commencement of germination; ct the cotyledons which 

 cohere at e above their elongated base, one of them with indi- 

 cation of a pinnate lamina B' at its apex, TV the primary root. 

 C germ-plant six months old ; sa denotes the seed, w the 

 primary root, b the first pinnate leaf, x, x the rudiments of 

 the lateral roots which subsequently grow upwards. 



[* Treub, in Ann. d. jard. Bot de Buitenzorg III, 1884, gives an account of the embryogeny in 

 Cycas circinalis. There is no ventral canal-cell ; an ovoid pro-embryonic mass of tissue enclosing 

 a central cavity is formed from the oospore, and the pluricellular suspensor with the embryo at its 

 extremity is developed from its lower end. The development of the embryo proceeds as in 

 other Gymnosperms.] 



