THE PHYSICAL BASIS OF LIFE 5 



eiitlv striietureless ground-substance or hjjalo plasm in 

 wliieli are suspended a great variety of visibly different 

 formed bodies, widely diverse in form and function, each 

 of which plays its own particular part in the activities of 

 the system. Examples of these bodies are, first of all, the 

 nucleus, and then the cytoplasmic chondriosomes and 

 plastids, the Golgi-bodies and central bodies and many 

 kinds of granules and fibrillae. The functions of these 

 various bodies are still imperfectly known; but all, un- 

 doubtedly, are centers of specific chemical activities 

 which contribute in one wav or another to the life of the 

 system. This fact is perhaps most clearly evident in case 

 of the plastids, especially characteristic of the cells of 

 plants, which are obviously specific centers for the forma- 

 tion of starch, chlorophyll and other pigments, and ap- 

 parently also in some cases of fats and proteins. The 

 nucleus plays an important part in growth and other 

 constructive processes ; it is jjossibly a center for the pro- 

 duction of enzymes ; and it is of fundamental importance, 

 tliouoh not the sole factor, in heredity. The central bodies 

 operate primarily as foci of cell-division; sometimes as 

 organs for the production of cilia or flagella ; and under 

 certain conditions as centers of aggregation for other 

 formed bodies. The granules often represent local accu- 

 mulation of storage-products of many kinds, such as food 

 materials, excretory j^roducts, pigment, pro-enzymes or 

 the like and a great variety of others; but, taken as a 

 whole, the granules may have a much broader significance 

 than this would seem to imply. The varied functions of 

 the fibrillae include contractility, conduction of stimuli, 

 and the support of the softer cell-structures; they also 

 often seem to play an important part in the processes of 

 secretion. The significance of the chondriosomes is not yet 

 definitely kno^vai. An important group of observers have 



LIBRARY -^ 



\ _ _ / 



