i8 



BOTANY 



a large number of very small pieces of living substance, 

 or protoplasm, each separated from its neighbours by a 

 thin membrane or cell wall which 

 surrounds it (Fig. 5). Very fine 

 connecting threads of protoplasm 

 extend through the cell walls and 

 so join the little pieces of proto- 

 plasm together, but these are so 

 delicate that it is not possible 



t0 See the With ut Vef y Skilful 



preparation. 



The living substance thus extends throughout the 

 plant in complete continuity, though it is apparently 

 divided into a number of separate pieces by cell walls or 

 membranes. These serve at the outset only for pur- 

 poses of support, and form a kind of 

 skeleton. Each little piece of proto- 

 plasm contains a small highly organ- 

 ised portion called its nucleus; the 

 whole piece is called a protoplast ; it 

 is approximately cubical in shape and 

 has a diameter of about i-30Ooth of 

 an inch. 



As the little protoplast absorbs 

 water and gets larger, entering into 

 active life, it finds itself in need of 

 constantly renewed supplies of water. 

 Here is its first individual difficulty, 

 for it is only the external cells 

 which can come into contact with 

 the water outside the plant. To 

 overcome this difficulty the proto- 

 plast gradually forms a central cavity 

 in its own substance, in which it 

 holds a store of water. This cavity is known as a 

 vacuole ; it is of the greatest importance in the 



FIG. 5. Vegetable cells. 

 h, celr.wall; p, proto- 

 plasm; k, k, nucleus; 

 s, vacuole. X7oo. 

 (After Sachs.) 



