150 The Living Plant 



mostly too small to be seen by the naked eye, but which appear 

 prominently in every magnified view of any part of any animal 

 or plant, as witness, for example, figm'es 2, 53, 73, 141, in this book. 

 We must therefore consider with some care the construction of 

 these cells, — a subject of the foremost importance in Biology. 

 The hairs earlier studied are fairly typical cells except that 



they are partially isolated from their 

 ^^.......y--^,-.^^^---:^.^ neighbors instead of deeply em- 



f^ \ ' 4 bedded among them, and are elon- 



\ ' ' ' ^^^^ , gated rather than rounded. If one 



i • r ■ '" cytoplasm '^ 



observes an example of these hairs, 



e. g., that of the Squash (figure 47), 



i ' t." .;. .: nucleolus he is Hkcly to notice first the clear- 



>■ ' _ ♦---piastid cut containing wall, inside of which 



— j.„_ sap-cavity comcs a Complete lining of soft gray- 



__jj/ granular protoplasm, very likely in 



^ slow streaming motion, with threads 



An optical section, highly ^f ^j^g g^j^g extending across the cell 



magnified, through a cell of the " 



Squash, showing all the parts of a at VarioUS aUglcS. This Soft proto- 



plasm is called cytoplasm. Some- 

 where within it, though not carried in its streaming, lies a denser, 

 rounded granular structure, also living protoplasm, the nucleus, 

 which often exhibits a round mass within itself, — the nucleolus. 

 In the cytoplasm lie also certain scattered granules (not especially 

 distinct, however, in these hairs), which are larger than food gran- 

 ules and otherwise unlike them; these too, are living protoplasm, 

 and are called plastids. Finally, within the cytoplasm appear 

 large open spaces, various in size and number but commonly 

 merged to a single very large one in old cells; though apparently 

 empty, they really are filled with a watery sap and therefore are 

 known as sap-cavities. These parts, wall, cytoplasm, nucleus, 

 plastids, sap-cavities, are the prominent parts of typical plant 

 cells, and the great majority of cells possess them all. We can 

 accordingly construct a conventionahzed cell showing these 



