204 COMPENDIUM OF GENERAL BOTANY. 



in the ' ' heterosporous ' ' cryptogams, whose sexually different spores 

 liav^e different prothallia. The larger spores, macrospores, produce 

 the archegonia, corresponding to the embryo-sac and egg-cell. Tlie 

 smaller spores, microspores .^ produce the fertilizing element, similar 

 to the pollen-tube of jDollen. Let us now consider the differences. 

 The macrospore becomes separated from the mother-jDlant and for 

 a time leads an independent existence during which it germinates 

 and produces archegonia. The emhryo-sac of gymnosperms, of the 

 phanerogams in particular, remains in union with the mother-plant. 

 It is enclosed by the coverings of the ovule, which usually appear 

 as the parts of a peculiarly modified leaf. The entire ovule as well 

 as the perfected pollen-grain are structurally arranged so that the 

 egg-cell in the embryo-sac may be formed and fertilized while still 

 in union with the mother-plant. After fertilization the egg-cell 

 undergoes division and develops into the embryo of the future plant. 

 Both the eml)ryo-sac and macrospore possess sufficient reserve food- 

 material to make the formation of tlie embryo possible. When 

 the embryo is fully formed, it becomes separated from the motlier- 

 plant and after a period of rest germinates and develops into a new 

 plant. It is possible to distinguish a neck and a Ijasal portion of 

 the so-called " archegonium " of gymnosperms, as in the arche- 

 gonium of ferns. 



The microspore is separated fi-om the mother-plant, likewise its 

 equivalent, the pollen-grain. However, the " germinating" micro- 

 spore of Selaginella, a vascular cr3q3togam, produces an entirely 

 different structure from that produced by tlie germinating pollen- 

 grain, although there is a n:iarked external similarity. In vascular 

 cryptogams fertilization takes place in water ; motile spermatozoids 

 with spirally wound plasmic cilia are formed. The pollen-tube, a 

 single non-septate slender filament, is formed from the pollen-grain. 

 Air-currents, insects, etc., carry the pollen-grain to the moist 

 stigma of the pistil, where it germinates, sending out the above- 

 mentioned pollen-tube, which penetrates the soft tissues of the 

 stigma and style, finally reaching the cavity of the ovary, where it 

 enters through the openings in the seed-coats (micropylar opening) ; 

 here it comes in contact with the apex of the body of the ovule and 

 embryo-sac, and finally the egg-cell. The morjjhological ajDpearance 

 of the developing pollen-tube is wholly different from the develop- 

 ment of the microspore. Their internal structure, which is also 



