176 



MANUAL OF BOTANY 



The apex of the tube comes to the summit of the macrospore, 

 where the necks of the archegonia are situated. It pierces the 

 coat of the spore and either enters the neck of a single arche- 

 gonium or spreads over several. A male gamete is extruded 

 from the tip into the oosphere or oospheres as the case may be, 

 and the nuclei of the two gametes fuse to form the nucleus of the 

 new zygote. 



This is the general arrangement throughout the group ; but 

 variations are found in some 

 Fig. 928. forms, particularly in the 



Gnetacese. 



The fate of the zygote differs 

 a good deal in the different 

 Natural Orders of the group. 

 The ConiferEe are the most 

 regular, and may be first dis- 

 cussed. The embryo is derived 

 from only part of the zygote ; 

 its nucleus goes down to the base 



, of the cell and divides into two, 

 arch. 



enSt. 



and each again into two ; the 

 four nuclei become four cells by 

 protoplasm aggregating round 

 them and ceU-walls being formed 

 between them. Each cell of the 

 four divides twice transversely, 

 so that instead of four cells there 

 are four tiers, each tier consist- 

 ing of three cells. Each of the 

 middle cells grows out into a 

 suspensor, the top cells of each 

 tier attach the suspensor to the 

 rest of the zygote, while the 

 terminal cells give rise to four 

 embryos. 



This mode of development 

 is that characteristic of the 

 AbietinesB ; in the Cupressinese a single tier of three cells is first 

 formed ; in Thuja only the two upper cells undergo longtitudinal 

 division, so that a single embyro is found, furnished with four 

 suspensor cells ; in Juniperus all three divide as in the Abietineas. 

 Picea cxceha of the Abietinese resembles Thuja in only forming 

 one embryo instead of four. In the Gupressinese the elongation 



Fig. 928. Macrosporangium (ovule) of 

 JHnus at maturity. After Dodel- 

 Port. mac. Macrospore. end. Ga- 

 metophyte or protliaUium. arch. 

 Archegonia. p.g. Pollen-grain or 

 microspore, whicli has been trans- 

 ported to the micropyle of the ovule 

 and has put out its prothallium, the 

 polleu-tube,^.^ 



