470 PART III. THE CLASSIFICATION OF PLANTS. 



gives rise to four microspores, which are usually tetrahedral, 

 bat bilateral in the Cycads. The dehisceiice is generally longi- 

 tudinal. 



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

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

 and Podocarpese) 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, Gnetacea?), 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, Podocarpese), or away from it (Cupressinese) : 

 they have a single integument, though in some genera (most 

 Taxoidece) an arillus is eventually developed. The macrospo- 

 rangia are indehiscent. 



The archesporium consists of one or more hypodermal cells of 

 the micropylar end of the nucellus : from the archesporium the 

 sporogenous cells are developed, as also tapetal cells. By the 

 formation of several layers of tapetal cells, and also by the re- 

 peated periclinal division of the micropylar epidermis, the 

 sporogenous cells come to be deeply placed in the nucellus, being 

 surmounted by a considerable mass of nucellar tissue which, in the 

 Cycadacea3, is hollowed out at the apex to form the pollen- chamber. 

 There may be a considerable mass of sporogenous cells (Cycadaceee, 

 etc., see p. 438), a condition which recalls that in the higher 

 Pteridophyta, or there may be a single sporogenous cell (Abietinese). 

 The sporogenous cell, or one of the sporogenous cells, grows 

 rapidly, causing the absorption of the adjacent cells, and is the 

 mother-cell of the macrospore : in some cases, where there are 

 many sporogenous cells, several of them may begin to grow in this 

 way, but as a rule, one gains the upper hand so that eventually 

 only one mother-cell is present. 



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 



