Ch. Ill, 5] 



PROTOPLASM 



41 



— WALL 

 PLASTID 

 SAKAVITY 



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, consistencjr, lying in the cyto- 

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

 ing upon the work of the cj'toplasm an influence which 

 guides the building of the organism along the general Unas 

 of its heredit}'. Inside the 

 nucleus is often a smaller 

 NUCLEOLUS, which con- 

 sists of a store of nutritive 

 matter used by the nu- 

 cleus. Third in promi- 

 nence in most plant cells 

 come the plastids, em- 

 bedded in the cytoplasm, 

 also of denser gelatinous 

 consistency, with rounded 

 or discoid forms. They 

 serve as seats of food for- 

 mation, the most prominent kind being the chloroplastids. 

 In some cells also, a fourth protoplasmic structure has been 

 newly recognized, viz., the very minute elongated bodies 

 called CHONDRiosoMEs or mitochondhia, 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, 



Fig. 1G. — A generalized plant cell, show- 

 ing the constituent parts, in optical sec- 

 tion. 



