I 



GROUP IV. PHAXEROGAMIA : GYMNOSPEKM^. 471 



the microsporangiate to the raacrosporangiate flowers, the Gymno- 

 sperms being anemophilous, and they come into direct relation with 

 the micropyle. 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 mucila- 

 ginous 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 Sporophyle. The Gymnosperms are all mero- 

 blastic and have a suspensor, with the single exception, in both 

 respects, of Ginkgo : they are frequently polyembryonic (most 

 Cupressineee, Abietineae, and Gnetacea^). 



The most peculiar type of development is that which is char- 

 acteristic of the Cycadaceae, of Ginkgo among Coniferce, and of 

 Ephedra among Gnetaceee. In these plants the germination of 

 the oospore begins with repeated nuclear division followed by 

 free cell-formation which leads, in the Cycadace83, to the production 

 either of a layer of cells round the wall of the archegonium (Cycas), 

 or of a mass of cells occupying its lower end (Ceratozamia), a 

 considerable cavity being left vacant in both cases : in Ephedra, 

 several (2-8) loose spherical cells are formed in a similar manner, 

 in the oospore, each with its own proper wall : and in Ginkgo, it 

 becomes completely filled with a mass of cells forming a compact 

 tissue. In the Cycadaceae, the embryonic cells at the lower end of 

 the oospore grow out into the endosperm, forming a suspensor at 

 the free end of which the body of the embryo is developed. In 

 Ephedra, each of the cells elongates into a suspensor which grows 

 out into the endosperm and produces an embryo. In Ginkgo, the 

 mass of cells constitutes the embryo itself ; there is no suspensor, 

 but the embryo, in the course of its growth, breaks out of the 

 archegonium into the endosperm. In the Coniferee (except Ginkgo) 

 the type of development is essentially the same throughout, thoug^h 

 with slight variations. In the Abietineoe 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 planes at right angles to each other, so that the 

 lower end of the oospore is occupied by a group of four cells lying 

 in one plane ; these cells then divide by transverse walls, so that three 

 tiers of four cells each are formed ; of these, each cell of the middle 

 tier grows out into a long unicellular suspensor ; those of the upper 



