Nichols, A morphological study of Juniperus communis var. depressa. 221 



to lie about the periphery of the young embryo sac, which 

 begins to enlarge and within three days has increased in 

 volume about twenty-five times (fig. 71). In the embryo sac re- 

 presented by this fignre sixteen free nuclei were counted. The 

 growth of the female gametophyte now proceeds rapidly, while 

 nuclear divisions continue to take place simultaneously, and by 

 May 25 th , just previous to the formation of cellular tissue, its 

 Volumetrie ratio, as compared with that of the megaspore at the 

 time of the first nuclear division, is about 12 500: 1. The embryo 

 sac has assumed the form of a prolate spheroid, the longitudinal 

 axis of which measures about 1400 jli. A figure of the entire cell 

 at the time of wall formation, if drawn to the same scale as 

 figs. 68, 70, and 71, would be 1.5 m. in length. Fig. 72 represents 

 probably the last free nuclear division, and in this preparation not 

 only are all of the nuclei undergoing division, but all are in the 

 same phase of mitosis. The axes of the spindles are for the most 

 part parallel with the major axis of the embryo sac. No attempt 

 was made to count the number of nuclei present after the last 

 free division. The number has been variously estimated in different 

 gymnosperms. Miss Ferguson (1904) writes that "about 2000 

 have been counted in Pinus Strobus at the time when the nuclei 

 are being separated by the development of dividing walls 1 ', and 

 aecording to Noren (1907) the number in -/. communis appears 

 to be scarcely sm aller. Growth still continues after the formation 

 of prothallial tissue, and at the time of fertilization the female 

 gametophyte has acquired a length of fully 2800 fi. 



The manner in which the cell tissue of the prothallium is 

 organized agrees closely for the most part with the observations 

 of Mlle. Sokolowa (1890) and Noren (1907) to which reference 

 has already been made. Immediately after the last free nuclear 

 division delicate anticlinal walls are laid down separating the 

 nuclei, but no periclinal walls are formed, so that the cells "appear 

 as uncovered boxes, the opening extending toward the center of 

 the prothallial cavity" (Ferguson 1904). When viewed from their , 

 inner, open ends the cells appear as in fig. 75. Their nuclei lie 

 slightly below the free inner margins of the walls, and from them 

 arise delicate Strands of cytoplasm which radiate toward the free 

 edges of the walls. An optical section through the nucleus and 

 parallel to the megaspore membrane (fig. 76) shows that these 

 radiations are restricted to the open ends of the cells, the re- 

 mainder of the cell cavity being intersected by coarser Strands of 

 cytoplasm. This relationship is even more clearly brought out in 

 a radial section (fig. 74). Here it is seen that the outermost 

 fibrillae are continuous from one nucleus to the next. As the walls 

 are secreted these Strands advance (fig. 7 7). so that, up to the 

 time when the inner ends of the tube-like cells become closed in, 

 all the nuclei of the embryo sac are connected with one another 

 by kinoplasmic fibrils. 



Aecording to Mlle. Sokolowa (1890) and Noren (1907) no 

 cross walls are formed until the tube-like cells meet at the center 

 of the prothallial cavity. Miss Ferguson (1904), however, finds 



