EMBRYOGENESIS IN GYMNOSPERMS 195 



So much for the single main embryo that comes to maturity. The 

 proembryo in Piniis may, however, give rise to four filamentous cleavage 

 embryos. Moreover, the four cells of the rosette remain meristematic 

 and each may give rise independently to a filamentous embryo. The 

 organisation of these embryos is like that of the distal embryos, but 

 soon — at about the twelve-cell stage — their further development is 

 restricted and they become abortive. Elongated suspensors are un- 

 common in rosette embryos. Well developed rosette embryos have been 

 observed in P. montana. A single fertilised egg may thus yield eight 

 young embryos ; and since three eggs are sometimes fertiUsed, a single 

 ovule may contain up to twenty-four embryos. Still larger numbers are 

 possible and have, indeed, been observed. Usually only one of these 

 embryos develops to maturity, though occasional seeds may yield two 

 fully formed embryos. 



Most species of Pinus have several archegonia, from two to six, 

 but in P. radiata and some other species, only one is present, this being 

 held to be an advanced and speciahsed condition. 



With modifications of detail, which can be referred to the size and 

 nature of the ovum, and to the distribution of growth in the embryo, 

 the foregoing account of the embryogeny of Pinus is applicable to 

 many of the Coniferales. 



Phylogenetic Aspect. Johansen follows Buchholz in the view that 

 species with a single, entire embryo have been derived from species in 

 which cleavage embryos were normally present, i.e. cleavage poly- 

 embryony was the primitive condition. In this view, Pinus probably 

 exemplifies the most primitive embryogeny among the Coniferales and 

 cleavage polyembryony may have originated in the ancestry of this 

 genus. This curious embryonic development, which is general in the 

 conifers, is modified in various ways in the more advanced members 

 of the group. The apical cell, which is a conspicuous histological 

 feature in the early embryogeny of Pinus, may be a character derived 

 from the original pteridophyte ancestors. 



Embryogeny and Classification. Classifications which were accepted 

 at an earlier stage have been modified in the light of later taxonomic, 

 anatomical and palaeontological studies. Buchholz has suggested a 

 classification based on embryology but so far this has not been co-opted 

 into any modern and generally accepted taxonomic system. Although 

 the Coniferales have many embryonic developments in common, 

 distinctive features occur in the several families and genera. The facts 

 of embryogeny may eventually enable botanists to indicate some of the 

 lines of evolutionary advance and thus afford a basis for a phylogenetic 

 classification. 



In the Pinaceae, Pinus is considered to exemplify the most primitive 



