84 MARGARET C. FERGUSON 



nuclei. This thin cytoplasmic shell encloses a large central 

 vacuole which is reported by Strasburger, Arnoldi and others 

 to be filled with a fluid substance. I have made no observa- 

 tions regarding the cell-sap of this large vacuole and can neither 

 affirm nor den}-^ its presence. 



The Second Pei'iod of Growth. — It has been seen that the 

 ovular development in Pinus is very slow during the period imme- 

 diately subsequent to pollination, but with the renewal of growth 

 in the spring development becomes much more rapid. Coor- 

 dinately with the enlargement of the ovule already described, 

 the endosperm cavity increases in size until it occupies almost 

 the entire basal and central portions of the nucellus, presenting 

 in longitudinal section the figure of an ellipse (fig. 71, plate 

 VI). The thin peripheral layer of cytoplasm with its free nuclei 

 persists until the latter part of May, and free nuclear division 

 continues to take place within it until a large number of nuclei 

 are formed. Jager ('99) estimated that there are 256 free nuclei 

 formed in Taxus, and Hirase ('95) made the same observation 

 in Ginkgo. The number is certainly much larger in Pinus. 

 More than 500 free nuclei are present early in May and about 

 2,000 have been counted in Pinus Strobus at the time when the 

 nnclei are being separated by the development of dividing walls. 



The free nuclei are considerably larger in surface view than 

 the nuclei of the nucellar tissue, but in side view they often 

 appear somewhat flattened. They have the structure of typical 

 resting nuclei (figs. 156-159, plate XV). Each contains, almost 

 invariably, two rather large nucleoli surrounded by clear areas. 

 The reticulum is close and studded with irregular granules, but 

 the net-knots are not so prominent as in the nuclei of the nucel- 

 lus. They simulate very closely the nuclei of the sheath-cells 

 at certain stages in the development of the archegonia. The 

 cytoplasm in surface view presents a pseudo-alveolar structure 

 consisting of a coarse, granular reticulum enclosing numerous 

 vacuoles (fig. 156). During the late telophase in the division of 

 the free nuclei of the prothallium the complicated karyokinetic 

 figure characteristic of free nuclear division becomes very con- 

 spicuous, and is evidently formed as a result of the rearrange- 

 ment of the cyto-reticulum (fig. 159). 



