i6 
INTRODUCTION 
matter. Many biologists now use the term protoplast 
(instead of cell) to designate the units of protoplasm. 
23. Structure of the Cell. Painstaking microscopic 
study of cells has revealed the fact that they have a wonder- 
fully beautiful and complex structure (Figs. 8 and 9). 
The protoplast is composed of two clearly denned parts, 
a denser, more or less globular portion, the nucleus, sur- 
rounded by cytoplasm (i.e., cell-plasm). Nucleus and 
FIG. 8. Cross-section of a cell from the root of a marrow-fat pea. 
n n , nucleolus; n.p nucleoplasm; n.m, nuclear membrane; a.m, starch- 
forming plastid; st, starch grain; c.w, cell- wall; c.p, cytoplasm; ch, chon- 
driosomes; they are scattered throughout the cytoplasm. (After D. M. 
Mottier.) 
cytoplasm together constitute protoplasm. The nucleus 
was discovered by Robert Brown, in 1831. The sub- 
stance of the nucleus is designated nucleoplasm, and there 
is generally a still denser body in the nucleus the nu- 
cleolus (plural, nucleoli). Sometimes there is more than 
one nucleolus within the nucleus. The most important 
chemical substance in the nucleus is chromatin, a very 
complex protein, rich in phosphorus. The name chromatin 
refers to the dense color it acquires when treated with 
