322 



ORGANOGRAPHY. 



Fig. 718. Fig. 719. consequent formation of a 



cavity of variable form and 

 size ; or it is produced (as 

 appears to be generally the 

 case) by the special develop- 

 ment of one of the cells of 

 the nucleus, which as it con- 

 tinues to increase in size, 

 presses upon the surround- 

 ing cells, and thus causes 



„.' „,..:,;, , J, ^^ „. ., . their more or less complete 



Fig. 71S. Undivided ovule of the Mistletoe , ,. rru- 



(Viscum album), consisting of a nak-ed absorption. IniS sac SOme- 



iiucleus. Fig. 719. The same ov.ule cut times causes the almost 



vertically to show the embryo-sac, c. , . , ^. j- ,•> 



n. Nucleus. entire absorption oi the 



nucleus, and even projects 

 beyond it, either through the opening in its coats afterwards to be 

 described, called the micropyle {fig. 722, m), or through its sides 

 in various directions, by which one or more saccate processes are 

 formed. The embryo-sac is surrounded by a thin layer of cells, 

 which has received the name of tercine. The sac contains at 

 first an abundance of fluid protoplasmic matter, in which before 

 impregnation (see Rkproduction or Angiospermia) three 

 nucleated cells, termed germinal vesicles, are usually developed. 

 Dr. P. Martin Duncan, however, in describing the process of 

 impregnation in Tigridia, states distinctly that previous to 

 that process taking place, the embryo-sac contains no granules 

 or cells, but simply colourless fluid. The protoplasmic semi- 

 fluid matter is by some called the liqtior amnios. Some ovules, 

 as those of the Mistletoe {fig. 719, c,n), consist simply of the 

 nucleus and embryo-sac as above described, in which case the 

 nucleus is termed naked. In almost all plants, however, the 

 nucleus becomes enclosed in one or two coats ; thus in the 

 Walnut, there is but one coat, which appears at first as a 

 little circular process around its base ; this gradually increases 

 in size, and by growing upwards, ultimately forms a sheath 

 or cellular coat to the nucleus, which it entirely closes except 

 at the apex, where a small opening may be always observed 

 {fig. 720). The coat thus formed, where there is but one, is 

 called the inUgmnentum simplex, s, and the orifice, end, at the 

 apex of the nucleus, n, is termed the micropyle or foramen. 

 Besides the Walnut, there is only one coat formed in the Com- 

 positse, Campanulacese, Lobeliacese, &c. 



In most plants, however, the ovule has two coats, in which 

 case we observe two circular or annular processes around the 

 base of the nucleus, the inner one being first developed, and 

 consequ^ently projecting at this time beyond the outer. These 

 processes continue to grow upwards as before described, until 

 they also ultimately form two sheaths or coats, which entirely 



