8 VEGETABLE PHYSIOLOGY 



some part of the lining layer itself. This body has a 

 more definite 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. Within this nuclear membrane are found 

 two substances which differ from each other in their power 

 of staining with various reagents. The bulk of the 

 nucleus is composed of a semi-fluid material known as 

 nucleoplasm, in which is embedded a network of fibrils or a 

 long much-coiled thread. The fibrils, or the thread, are 

 composed of a hyaline substance in which lie, close to each 

 other, a number of granules which stain deeply with many 

 colouring matters. The threads contain these granules 

 in such large proportion, that, except with very high magni- 

 fication, the latter cannot be distinguished, and consequently 

 the whole fibril appears stained. The fibrils are generally 

 said to be composed of chromatin, the name having refer- 

 ence 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 or the crossings of the 

 coiled-up thread (figs. 6, k k, and 9, b). Chemically the 

 nucleus resembles the rest of the protoplasm to a consider- 

 able extent. It contains, however, a material known as 

 nuclein, of which phosphorus is a constituent. It is not 

 known how the nuclein is related to the rest of the nuclear 

 substance, but it appears to be present in the thread or 

 fibrillar network arid not in the general nucleoplasm. 



It is of such protoplasts or aggregations of small 

 portions of living substance that all plants are built up. 

 There is, however, a wonderful variety in the relative 

 arrangements of these units of construction, a variety 

 which finds its expression in the multiplicity of existing 

 forms, and the differences of dimensions which various 

 organisms exhibit. 



