16 BOTANY. 



cell-wall ; and (2) within this a less dense granular one, the 

 endoplasm ; the two layers are, however, not separated from, 

 each other by any sharp line of demarkation. * 



When the endoplasm attains a considerable thickness it becomes dif- 

 ferentiated into an external denser layer and an internal less dense 

 one. Often one of these layers may be found to be in motion while the 

 other is at rest.f 



15. There may almost always be seen in plant-cells bands 

 or strings of protoplasm which lie in or between the vacu- 

 oles (Fig. 2, J5). They are at first thickish plates which 

 separate vacuoles, but afterward they become narrower as 

 the vacuoles enlarge, and at last they disappear entirely. In 

 these bands and strings, as previously stated (paragraph 12), 

 streaming movements are frequently to be seen. 



16. Each of the protoplasm masses constituting the cells 

 of most plants usually has a portion of its interior substance 

 differentiated into a firmer rounded body, the nucleus Its 

 normal position is in the centre of the cell ; but it may be 

 displaced and pushed aside by the vacuoles, so that in an 

 optical section of the cell it may often appear to be in the 

 margin. The nucleus is to be regarded simply as a modified 

 part of the protoplasm of the cell, and not as something dis- 

 tinct from it. It may dissolve, and its substance pass into 

 that of the remainder of the cell ; afterward a nucleus may 

 form again ; and this may occur a number of times. Com- 

 monly in each nucleus one or more small rounded granules 

 may be seen ; these are called the nucleoli. The nucleus 

 may form a skin (liautschicht) about itself, and vacuoli may 

 be present in its interior. 



17. Cells are of very varying sizes. They differ in dif- 

 ferent plants, and also in the different parts of the same 

 plant. In but few cases, however, are they of great size, by 

 far the larger number being microscopic. The most striking 



* These two layers were first described by Pringsheim in his " Theorie 

 der Pflanzenzelle," 1854. 



f Cf. Strasburger, " Studien liber Protoplasma," 1876 ; and Qr. Jr. 

 Mic. Science, 1877, pp. 124-132. 



