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, 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 and 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. 
