424 PART IV. CLASSIFICATION. 



cells also derived from the archesporium, and by a wall consisting 

 of one, two, or more, layers of cells : each, spore-mother-cell gives 

 rise to four microspores, which are usually tetrahedral, but 

 bilateral in the Cycads. The dehiscence is generally longitudinal. 



The microspores (pollen-grains) present no special features be- 

 yond the fact that in some genera of Coniferae (e.g. most Abietineae) 

 the exine is dilated into two hollow expansions which lighten the 

 pollen-grains and facilitate their dispersal by the wind. 



The macrosporangia (ovules) are borne either terminally on a 

 floral axis (e.g. Taxese, Gnetaceae), or on the upper surface of a 

 macrosporophyll ; on the floral axis they are borne singly, on the 

 sporophylls their number varies (1-7) : they are orthotropous and 

 sessile, the micropyle being directed either towards the axis of the 

 cone (in Abietineae), or away from it (Cupressineae) : they have a 

 single integument, though in some genera (most Taxoideae) au 

 arillus is eventually developed. 



The macrospore (embryo-sac) is developed singly in the macro- 

 sporangium, by the growth and maturation of the mother-cell 

 which does not undergo division into four as in the Pteridophyta. 

 In the Cycadacese the wall of the macrospore, like that of spores 

 generally, is differentiated into two layers, the outer of which is 

 cuticularised. 



Pollination. The microspores are conveyed by the wind from 

 the microsporangiate to the macrosporangiate flowers, the Gymno- 

 sperms being anemophilous, and they come into direct relation with 

 the ovule. In the case of cone- flowers, the scales separate at the 

 time of pollination, to permit of the pollen-grains being blown in 

 between them. The micropyle of the ovule secretes a mucilaginous 

 liquid which catches one or more of the pollen-grains : by the 

 gradual evaporation of this liquid, the pollen-grain is drawn 

 down the micropyle and is lodged on the apex of the nucellus, 

 where it germinates. 



Embryogeny of the Sporophyte. The Gymnosperms are 

 remarkable in that they are frequently polyembryonic (most Cu- 

 pressineae, Abietineae, and Gnetacese), though the ripe seed eventually 

 contains only a single embryo (see p. 401). 



In the Coniferae (except Ginkgo) the type of development is 

 essentially the same throughout, though with slight variations. 

 In the Abietinese the nucleus of the oospore descends towards the 

 lower end of the cell, and divides into two, and each of these again 

 into two; cell-formation takes place, walls being formed in two 



