292 



MORPHOLOGY OF GYMNOSPERMS 



reports that four free nuclei (in one case six) pass to the bottom. 



There is agreement that the fusion nucleus of Taxodium passes to 



the bottom of the egg before division; while 

 in Cunninghamia this position is taken at 

 the two-nucleate stage, and in Sciadopitys 

 at the four-nucleate stage (as in Pinus). 

 The indications are that this movement 

 occurs at any time before wall-formation, 

 but most frequently after two divisions. 



In general, also, the completed pro- 

 embryo consists of three tiers of cells 

 instead of the four observed in Pinus, the 

 "rosette" tier not being present. The tiers 

 may not contain the same number of cells, 

 as the proembryo of Taxodium will illus- 

 trate. Preceding wall-formation the eight 

 nuclei are arranged in two tiers, the upper 

 containing six cells, the lower two. There 

 is only one division of tiers after wall-for- 

 mation, which divides the upper tier into the 

 suspensor cells and the open cells; at the 



same time, the two basal cells divide by vertical walls to a tier of 



four cells. The completed proembryo, 



therefore, consists of six open cells, six sus- 

 pensor cells, and four embryo cells. 



The case of Sequoia sempervirens (92) 



is very peculiar among Pinaceae. There 



is no free nuclear division at all, a wall 



being formed at the first division, which 



results in two cells nearly filling the egg 



(fig. 356). Succeeding divisions result 



in a linear row of five large cells, the 



lowest forming the embryo, and the next 



upper one elongating to form the sus- 

 pensor. The elimination of the free nuclear 



stage and the complete occupation of the 



egg by a few embryonal cells are facts difficult to relate at present. 



Fig. 356. — Sequoia sem- 

 pervirens: first division of 

 the fertilized egg, showing a 

 wall forming between the 

 two daughter nuclei; X500. 

 — After Lawson (92). 



Fig. 357. — Sciadopitys 

 vcrticillaia: secondary sus- 

 pensors. — After Arnoldi 



(64). 



