56 



BOTANY 



PAKT I 



Fig. 54. — Quiescent nucleus from 

 the developing endosperm of 

 FritillaHa imperialis, hard- 



nuclear membrane ; c, cen- 

 trospheres. (Somewhat dia- 

 grammatic, x 1000.) 



living objects can only be distinguished by the punctated appearance it 

 gives to the nucleus. Streaming movements do not take place within 

 the nucleus. An insight into the nuclear 

 structure is only to be attained with the help 

 of properly fixed and stained preparations. It 

 is then possible to determine that the greater 

 part of this nuclear network is composed of 

 delicate and, for the most part, unstained 

 threads, in which lie deeply stained granules. 

 The substance of the threads has been distin- 

 guished as linin (I), that of the granules as 

 chromatin (ch). One or more large nuclear 

 bodies, or nucleoli (?i), occur at the inter- 

 sections of some of the linin threads which, 

 although deeply stained, have not taken the 



ened with alcohol and stained game tinfc ag t ^ e c ] iroma tin granules. The 

 with safranin, I, Linin ; ch, . 1 . .,-, ■ ,-i 



chromatin; n, nucleolus ; w, network of the nucleus lies witnm tne 

 nuclear cavity, which is filled with nuclear 

 sap and surrounded by a membrane (w). 

 Although this is generally spoken of as the 

 nuclear membrane, strictly speaking it is a part of the surrounding 

 cytoplasm, and is the protoplasmic layer or pellicle with which the 

 cytoplasm separates itself from the nuclear cavity. 



The Centrospheres. — The existence of these bodies, now universally 

 acknowledged in animal cells, is generally ad- 

 mitted in the case of all vegetable cells, although 

 their demonstration has not, in all cases, been 

 successful. They form, as Guignard in parti- 

 cular has shown, two small homogeneous spheres 

 lying near the nucleus and embedded in the 

 cytoplasm. Each centrosphere has in its centre 

 a body termed the centrosome (c, Figs. 50, 54), 

 composed of one or more small granules. As 

 the successful fixing and staining of the centro- 

 spheres in vegetable cells require extreme care, 

 their detection in the granular cytoplasm is 

 rendered difficult. 



The Chromatophores. — In the embryonic 

 cells of growing points, where the chromatophores 

 (Fig. 50, ch) are principally located around the 

 nucleus, they first appear as small, colourless, fig. 55.— Two ceils from a 

 highly refractive bodies ; and in the embryonic leaf o£ J « a "« »»"- 



?, j. 1 ,1 1 • -i metrics, tf , Chloroplasts : 



cells of ovules they have a similar appear- n< nucleus. ( x soo.) 

 ance. They may retain the same appearance in 

 older cells (Fig. 104, J, I), but in them they also attain a further 

 development. Chloroplasts, leucoplasts, or chromoplasts may 



