474 TART III. THE CLASSIFICATION OF PLANTS. 



their end the cells of the lowest tier which develope into but a 

 single embryo, whereas in the typical Abietiuese four embryos 

 originate from each oospore. 



Among the Cupressinese, the first step is the formation of three 

 cells one above the other at the lower end of the oospore : in 

 Thuja occidentalis only the two upper cells undergo longitudinal 

 division, and, consequently (as in Picea excelsa) only a single 

 embryo is developed from each oospore : in Juniperus, all three 

 cells undergo longitudinal division, so that four embryos are here 

 developed from each oospore. In this group the cells of the 

 uppermost tier elongate to form the suspensors : those of the 

 middle tier contribute to the length of the suspensors, and also 

 give rise to short embryonal tubes which invest the suspensor for 

 some distance, as also the base of the embryo : the cell or cells 

 of the lowest tier give rise to the embryo (Thuja) or embryos 

 (Juniperus). 



In the Taxeas, as represented by Taxus and Cephalotaxus, the 

 embryogeny closely resembles that of Thuja, though there is not 

 the same degree of constancy ; two or three tiers of cells, varying 

 in number, are formed at the lower end of the oospore ; of these, 

 the uppermost tier (Taxus) or second tier (Cephalotaxus) grow out 

 into the (usually six) suspensors ; those of the lower or lowest tier 

 form the single embryo and the embryonal tubes. 



The embryogeny of the Gnetaceous genera Gnetum and Welwit- 

 schia is peculiar : in Welwitschia the oospore, whilst still enclosed 

 in the wall of the archegonium, undergoes a transverse division at 

 its base, which cuts off a small flat cell as the embryo- eel I ; the 

 wall of the archegonium continues to grow, and forms the sus- 

 pensor ; the embryo-cell gives rise to the single embryo and to a 

 number of embryonal tabes which invest the lower portion of tho 

 suspensor : in Gnetum, so far as the embryogeny is known, it 

 appears that an embryo-cell is formed at the end of the suspensor, 

 as in Welwitschia, from which the embryo and the embryonal tubes 

 are also developed ; the suspensor appears to branch, each branch 

 bearing an embryo. 



The growing-point of the stem is developed at the anterior end 

 of the embryo: two species of Coniferae (Cephalotaxus Fortunei, 

 Araucaria brasiliana) present the peculiarity that the growing- 

 point is not quite terminal, but is covered by a small group of cells 

 which are subsequently thrown off : in some cases (Cupressinece, 

 Pinus Strobus, apparently Taxus and Gnetum, and occasionally 



