LIFE HISTORY OF PINUS 8 1 



dition. I now have an abundance of preparations which show 

 not only that the wall layer is instituted in the one-celled stage, 

 but that it persists as long as free cell-formation continues in the 

 endosperm. The only reference which I find regarding the 

 establishment of the wall-layer of cytoplasm in any of the 

 Gymnosperms is the following statement made by Coulter and 

 Chamberlain ('01), with reference to Pinus: " Probably when 

 but two or three free nuclei have appeared the nuclei become 

 imbedded in a parietal, cytoplasmic layer." 



SUMMARY. 



The female cones can be distinguished early in March, 

 excepting in Pinus Strobus where they do not appear until the 

 very last of April. The ovules cannot be detected until about 

 three weeks before pollination. 



There is no evidence that the macrospore-mother-cell arises 

 from a hypodermal cell. When first differentiated it is cen- 

 trally placed nearer the chalazal end of the ovule. 



The division of the macrospore-mother-cell is a true tetrad- 

 division and the cell which gives rise to the female gametophyte 

 is a true spore. 



Of the two cells formed as a result of the heterotypic division 

 the lower one always divides again, the upper one may. An 

 axial row of three cells seems to be the rule in Pinus Strobus 

 and P. rigida, and one of four cells the rule in P. austriaca, 

 though neither is constant in any of the species. The lowest cell 

 of the axial row always becomes the functional macrospore. 



The two or three upper cells of the axial row begin to disin- 

 tegrate very soon after they are formed and are finally absorbed 

 by the enlarging macrospore. 



The lower cell passes through a long period of growth during 

 which the cytoplasm is withdrawn from the central portion of 

 the cell and forms a uniform layer lining the wall of the macro- 

 spore. The nucleus moves towards the upper side of the cell 

 and imbeds itself in the peripheral layer of cytoplasm. 



The suggestion is made that the embryo-sac may or may not 

 be a morphological unit, but that it is essentially a physiological 

 unit, existing for the purpose of sexual reproduction. Such a 



