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 (Figc. 8 and 9). 

 The protoplast is composed of two clearly defined 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. 

 M", nucleolus; n.p nucleoplasm; n.m, nuclear membrane; a.m, starcli-. 

 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 



