EMBRYOGENY OF PhiHs Pinaster. 55 



will a little later form the growing region of stem apex, cotyledons 

 and plerome. 



The position of the embryo of figure 12 in the prothallus is 

 indicated io figure 11. together with two secondary embryos, 

 which would probably not persist much longer. The position of 

 the embryo of figure 10 and two secondary embryos is also indicated 

 diagrammatically to the same scale on the right of the same figure. 

 Other features shown in this figure have already been discussed. 



Figures 13 and 14 show the outline and structure of an embryo 

 when the position of the cotyledons is first indicated. As would 

 be expected they arise by the increased activity of as many masses 

 of apical meristem as there will be cotyledons. If, however, the 

 polycotyledonous condition in conifers has arisen from a primitive 

 dicotyledonous* type, as claimed by Hill and de Fraine (8) (9), 

 one would rather expect to find two masses of meristem first 

 making their appearance and later becoming differentiated into 

 separate masses, but there is not the slightest indication that this is 

 so in Pi II us pinaster. Transverse sections of embryos of similar 

 age to the one here figured show each cotyledon rudiment develop- 

 ing to an equal extent. 



At this stage the whole tissue of the embryo is still meristematic, 

 but the line of junction between proximal and distal meristem 

 (about the middle of figure 14) is more clearly marked than in 

 figure 12. Thus we see that about four-fifths of this embryo is the 

 peribl m of the root and the other one-fifth stem, cotyledon and 

 plerome meristem, these regions not being separated irom one 

 ■another by any " permanent " tissue. The origin of the root 

 cap from the periblem has been accurately described by De Bary (6) 

 and its position is indicated in figures 13 and 15. 



The last stage figured (figure 15) shows an embryo somewhat 

 older, in which the cotyledons are clearly defined. The outline 

 of the different tissues is indicated in the same way as in figure 13. 

 The distal meristem has now become much more clearly differ- 

 entiated into three distinct parts : (i.) the cotyledons, (ii.) the stem 

 apex, and (iii.) the plerome. The latter merges gradually into the 

 stem apex, but at a somewhat later stage, when branches of the 

 plerome extend into the cotyledons the two are sharply separated. 

 The early appearance of a well-marked plerome in Conifers is 

 conspicuously unlike w'hat obtains in Cycads and Ginkgo, where 

 only one kind of root meristem (no doubt representing the periblem) 

 is to be recognised in the embryo. 



Besides the investigation here reported, the writer has followed 

 many of the important stages in the life history of this species, 

 •esperially pollination, the development of the gametophytes, 

 fertilisation and development of the proembryo. 



The occasional occurrence of parthenogenesis (in the sense in 

 Avhich Strasburger now uses the term) has already been reported 

 by the writer (14). In other respects the life history is very similar 

 to that reported by Blackman (I), Coulter and Chamberlain (5), 



*It is perhaps well to point out that the word ' dicotyledonous" is heri: 

 used in a purely literal sense and has no reference to the Angiosperms. 



