LIFE HISTORY OF PINUS 



93 



The nucleus of the central cell attains full size very soon 

 after its formation. It has a delicate, more or less interrupted 

 reticulum, and is characterized by a large vacuolate nucleus 

 which invariably occupies a central position. One or two 

 smaller nucleoli may also be present. This nucleus always 

 remains close beneath the neck-cells, as is the case in other 

 Gymnosperms, and, as a rule, is more or less concave on the 

 side toward these cells (figs. 172-177,181-183). As Blackman 

 has pointed out, the vacuolate nature of the cytoplasm renders 

 this nucleus very liable to displacement during the early stages 

 in the development of the archegonia, yet with well fixed ma- 

 terial it is always found in its normal position. Hirase ('95) 

 states that certain granules, which appear in the cytoplasm just 

 beneath the nucleus of the central cell in Ginkgo, have been 

 derived from this nucleus or from its nucleolus. Ikeno ('98), 

 also, describes the nucleus of this cell in Cycus as giving out a 

 granular substance during its growth period. No comparable 

 phenomenon has been observed in connection with the nucleus 

 of this cell in the species of pines which I have studied, but, as 

 above stated, the nucleus quickly reaches its mature size and 

 remains apparently unchanged until the inception of its division. 



Very early in the history of the archegonium, the cells imme- 

 diately surrounding it become differentiated from the adjacent 

 endosperm-cells by their more regular form, the greater density 

 of their cytoplasm, and the increase in the size of their nuclei. 

 Thus a distinct sheath is formed about the venter of the arche- 

 gonium. This sheath usually consists of a single layer of cells. 

 It is more conspicuous in Plnus resinosa than in the other species, 

 and may become two cells broad at certain points, but even here 

 it is never two layered to any considerable extent. The nuclei 

 of these cells divide as the archegonium increases in size, the 

 axes of the spindles being always parallel with that face of the 

 cell which is adjacent to the egg. All the sheath-cells of a 

 given archegonium have several times been observed in the 

 same stage of mitosis, but this is very exceptional as these cells 

 do not ordinarily divide simultaneously. The sheath-cells 

 persist until after fertilization when they gradually lose their 

 cytoplasm and resemble the other cells of the prothallium. 



