136 A TEXT-BOOK OF BOTANY. 



associated with the nucleus and appear to be concerned in cell 

 division. There are, in fact, quite a number of minute bodies in 

 the cytoplasm which may be always present or only under certain 

 conditions, and which are grouped under the general name of 



MICROSOMES or MICRQSOMATA. 



Chemically protoplasm is an extremely complex substance, but 

 does not appear to have a definite molecular structure of its own, 

 being composed in large measure of proteins, a class of organic 

 compounds which always contain nitrogen, and frequently phos- 

 phorus and sulphur. The molecule of the proteins is large and 

 more or less unstable, and hence subject to rapid changes and a 

 variety of combinations, and it is to these interactions that the 

 vital activities of the plant are attributed. 



Nucleus. — The nucleus consists of (i) a ground substance 

 in which is embedded (2) a network composed of threads con- 

 taining a granular material known as chromatin, and (3) gen- 

 erally one or more spherical bodies called nucleoles, the whole 

 being enclosed by (4) a delicate membrane (Fig. 85). The chro- 

 matin threads are readily stained by some of the aniline dyes, and 

 are mainly composed of nucleins (proteins) rich in phosphorus, 

 which by some writers are supposed to be essential constituents of 

 the nucleus and necessary to the life of the protoplast. Chroma- 

 tin is constant in the nucleus, and prior to cell division the threads 

 become organized into bodies of a definite number and shape 

 known as chromosomes (Fig. 85, s). 



Plastids. — The plastids or chromatophores form a group of 

 differentiated protoplasmic bodies found in the cytoplasm (Front- 

 ispiece) and are associated with it in the building up of complex 

 organic compounds, as starch, oil, and proteins. The term chro- 

 matophore means color-bearer, but applies also to those plastids 

 which may be colorless at one stage and pigmented at another. 

 Hence we may speak of colorless chromatophores. According 

 to the position of the cells in which these bodies occur and the 

 functions they perform, they vary in color — three distinct kinds 

 being recognized, (i) In the egg-cell and in the cells of roots, 

 rhizomes, and seeds the plastids are colorless and are called leuco- 

 PLASTiDS. (2) When they occur in cells which are more or less 

 exposed to light and produce the green pigment called chloro- 



