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 defined parts, 

 a denser, more or less globular portion, the nucleus, sur- 

 rounded by cytoplasm {i.e., cell-plasm). Nucleus and 



r^Wl^mk--^^ 



Fig. 8. — Cross-section of a cell from the root of a marrow-fat pea. 

 w", 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 



