154 



BOTANICAL GAZETTE 



[FEBRUARY 





cut off by complete walls from the rest of the cytoplasm of the egg. 

 Each cell of this tier constitutes an initial cell to one of the 4 primary 

 embryos. The tier above it is not completely walled, and therefore 

 undergoes another free nuclear division, organizing the second tier of 

 completely walled cells (r), the rosette tier, a group of initial cells 

 of the rosette embryos. The open tier of free nuclei (0) which 

 remain above this undergo no further division and soon disintegrate. 

 When these 3 tiers of 8 walled cells and 4 free nuclei have formed, 

 as in fig. 5, the organization stage of the proembryo is concluded, 

 for each cell is now ready to produce its own distinct embryo, 

 although the 4 cells of the primary embryo initial tier (p) continue 

 their further development in unison. 



Figs. 1-5. — Steps in development of proembryo in Pinus, diagrammatic recon- 

 structions from serial sections and published figures: p, tier of primary embryo initial 

 cells; r, tier of rosette cells, initial cells of rosette embryos; 0, upper open tier of cells; 

 normally tiers r and o come from division (free nuclear) of upper tier of fig. 4. 



From each of these 8 completely walled embryo initials (fig. 5) 

 an embryo develops by means of an apical cell, this cell functioning 

 first as a hemispherical apical cell of one cutting face, and later as 

 a semi-pyramidal cell of 3 cutting faces, in a manner described 

 in greater detail elsewhere (3). It may be added that this apical 

 cell persists until an embryo mass of about 500 cells has been 

 formed, after which it is replaced by the meristematic group of 

 cells found in the older conifer stem tip. This apical cell is a primi- 

 tive feature in which conifers recapitulate their fern phylogeny. 



The early embryo of Pinus. — The cells (p) of the embryo 

 initial undergo simultaneous division, in which their first apical 



