26o 



MORPHOLOGY OF GYMNOSPERMS 



(87) estimated that in cases of Pinus under her observation 2,000 

 free nuclei were produced before wall-formation. This would mean 

 approximately eleven successive divisions, but there is no reason to 



suppose that the regularity of the 

 early divisions is continued by the 

 later ones. This period of free 

 nuclear division may be spoken 

 of as the first stage of endosperm 

 development. 



The second stage of develop- 

 ment is that of wall-formation, 

 during which walls appear among 

 the free and usually equally dis- 

 tributed nuclei, and a parietal 

 layer of cells is formed, which are 

 open toward the center of the 

 sac. The third stage is that of 

 centripetal growth, the parietal 

 tissue growing toward the center 

 of the sac from every direction, 

 until the sac becomes filled up 

 with tissue. In this centripetal 

 growth the innermost cells are 

 always open toward the center, 

 and may become quite elongated. 

 When the center has been reached 

 by the abutting cells, end walls 

 are formed by each open cell and 

 the tissue is continuous through- 

 out the sac. It will be noted 



Fig. 284. — Pinus Strobus: longi- 

 tudinal section of ovule, May 26, 1898; 

 gametophyte in the free nuclear condi- 

 tion, and surrounding it is a jacket of 

 spongy tissue mostly two cells thick; 

 X62. — ^After Miss Ferguson (87). 



that this outline is in general that which was given for Cycadales 

 and Ginkgoales. 



The details of the centripetal growth have been found to vary 

 widely, but this growth has been studied in a close series in very few 

 forms, so that no general conclusions are possible as yet. The case 

 of Sequoia sempervirens was first described by Aenoldi (49) and 

 later by Lawson (92). Free nuclear division occurs in the two 



