Ch. Ill, 5] PROTOPLASM 41 



Most abundant, though often not most prominent, is the 

 gelatinous-mobile cytoplasm, which is clearly the working 

 part of the cell, — that which transports materials, builds 

 the wall, produces chemical reactions, and the like. Next in 

 prominence is the nucleus, a rounded body of denser but 

 still gelatinous, or colloidal, consistency, lying in the cyto- 

 plasm. It seems clearly the control organ of the cell, exert- 

 ing upon the work of the cytoplasm an influence which 

 guides the building of the organism along the general lines 

 of its heredity. Inside the 

 nucleus is often a smaller ^mm^mm^^^--. 



nucleolus, which con- /M W plastid 



sists of a store of nutritive 

 matter used by the nu- 

 cleus. Third in promi- ^ 

 nence in most plant cells 



come the plastids, em- W$/-~~ ; nucleolus 



bedded in the cytoplasm, - cytoplasm 



also of denser gelatinous p 



Consistency, with rounded FlQ . i 6 . _ A generalized plant cell, show- 

 Or discoid forms. They in S tn e constituent parts, in optical sec- 



serve as seats of food for- 

 mation, the most prominent kind being the chloroplastids. 

 In some cells also, a fourth protoplasmic structure has been 

 nevvly recognized, viz., the very minute elongated bodies 

 called chondriosomes or mitochondria, as to the nature 

 of which, however, we as yet know little. 



Such are the protoplasmic parts of the typical plant cell. 

 In addition, most cells possess a firm wall, built by the 

 cytoplasm, and composed of a firm-elastic water-permeable 

 substance called cellulose. The wall has the obvious func- 

 tion of a support to the protoplasm, which is far too soft to 

 support itself; and the collective walls of all the cells con- 

 stitute a firm skeleton for the plant. In young and small 

 cells the protoplasm completely fills the space within the 

 wall, but as they grow older and larger, rifts, filled with sap, 



