284 MANUAL OP BOTANY 



this motility is, of oovirse, less and less obvious. Indeed, in most 

 cells it can hardly be distinguished. There is reason to suppose, 

 however, that protoplasm, wherever existing, is in active motion. 

 In many of the constituent cells of even the higher plants this 

 motility can be observed. In certain of the cells forming the 

 leaves of many water-plants, e.g. Vallisneria, Nitella, Elodea 

 (fig. 614), and others, a streaming movement of the granules the 

 protoplasm contains can be detected with ease. In other plants 

 of terrestrial habit, such as Tradescantia and Chelidonium, a 

 streaming of the protoplasm is observable (fig- 615). Such 

 movements are spoken of as rotation when the stream flows 

 uniformly round the wall of the cell, or as circulation when the 

 path has a more complicated course. 



Somewhere in the cell there is always to be found a specially 

 differentiated portion of the protoplasm known as the nucleus 

 (figs. 614 and 615, n). It may lie in the centre, when it is attached 

 to the sides by bauds or threads of protoplasm ; or it may lie 

 embedded in the layer which lines the cell. This body has a 

 more deiinite structure than the rest of the cytoplasm ; it is 

 bounded at the surface by a delicate membrane, which is thought, 

 however, to be a denser layer of the protoplasm of the cell 

 rather than to belong to the nucleus itself; it consists of two 

 substances which differ from each other in their power of 

 staining with different reagents. The bulk of the nucleus is 

 composed of a semi-fluid material known as nucleoplasm, in 

 which is embedded a network of fibrils. The latter are com- 

 posed of a hyaline substance in which lie close to each other a 

 number of granules which stain deeply with many colouring 

 matters. The fibrils contain the granules in such large propor- 

 tion that except with very high magnification the latter cannot 

 be distinguished, and consequently the whole fibril appears 

 stained. These fibrils are generally said to be composed of 

 chromatin, the name having reference to nothing more than 

 this reaction to stains. 



One or more small deeply staining bodies, termed nucleoli, are 

 found in each nucleus, sometimes being very prominent, and at 

 other times hardly distinguishable from the nodes of the fibrillar 

 network. 



Chemically the nucleus much resembles the rest of the 

 protoplasm, but, in addition to the substances found to be present 

 in the latter, the nucleus contains a body known as nuclein, of 

 which phosphorus is a constituent. 



Associated with the nucleus there have been observed in 



