CONIFERALES 83 



that given for Cycads. The megaspore nucleus divides, and 

 this is followed by repeated simultaneous nuclear divisions 

 until a large number of free nuclei have been organized. 

 According to Jager, 40 about 256 free nuclei appear in Taxus 

 baccata before walls are formed. This represents eight suc- 

 cessive divisions, and coincides exactly with Hirase's estimate 

 of the free nuclei in the embryo sac of Ginkgo previous to 

 the formation of Avails. Very early in this series of divisions, 

 probably in general when but two or four free nuclei have ap- 

 peared, the nuclei become imbedded in a parietal cytoplasmic 

 layer inclosing a large central vacuole (Fig. 62), and in this posi- 

 tion free nuclear division is completed. Subsequent divisions of 

 the nuclei are accompanied by the formation of cell walls and a 

 parietal tissue is organized. From the parietal plate of cells 

 the cavity of the embryo sac is gradually filled with tissue, the 

 faces of the cells directed toward the center in this advance lack- 

 ing walls. The more compact tissue toward the micropyle or- 

 ganizes archegortia, while the deeper tissue is nutritive. 



A deviation from this method of endosperm formation has 

 been reported by Arnold 42 as occurring in Sequoia. After the 

 development of the free nuclei, the cytoplasmic layer increases 

 in thickness, accumulating especially at the lower end of the sac. 

 Then cell formation occurs in the upper and lower extremities of 

 the sac, while free nuclear division continues in the central 

 region. 



About the last of May the archegonium initials become ap- 

 parent. "Within a week the primary neck cell has been cut off, 

 and the central cell has become much enlarged (Fig. 64, A, B). 

 As the archegonia develop, the adjacent endosperm tissue con- 

 tinues to grow, so that a more or less evident depression appears 

 over each archegonium in case they are scattered (Abieteae), or 

 over each group of archegonia in case they are clustered (Cupres- 

 seae and Taxodieae). The number of archegonia developed by 

 a single gametophyte varies considerably. In Abieteae there are 

 three to five; in Taxus baccata five to eleven; in Cupresseae five 

 to fifteen, or even thirty. The neck of the archegonium shows 

 much variation. In Cycads and Ginkgo the primary neck cell 

 divides but once, by an anticlinal wall, forming a neck of two 

 cells, placed side by side as are the guard cells of a stoma. In the 

 majority of Conifers observed a plate of four cells is derived 



