16 



VEGETABLE PHYSIOLOGY 



sufficiently coarse to be visible under a comparatively low 

 power of the microscope, and to need hardly any special 

 preparation (fig. 18). 



It will no doubt have been noticed that the term ' cell ' 

 is somewhat loosely used. A typical cell of a multicellular 

 plant consists of three parts the protoplast, the cell-wall, 

 and the vacuole (fig. 6) ; of these the first is the most 



FIG. 17. CONTINUITY OF THE PBOTOPLASM 

 OF CONTIGUOUS CELLS OF THE ENDOSPEBM 

 OF A PALM SEED (Bentinckia). Highly 

 magnified. (After Gardiner.) 



a, contracted protoplasm of a cell; b, a 

 group of delicate protoplasmic filaments 

 passing through a pit in the cell-wall. 



FIG. 18. SEMI-DIAGRAMMATIC LON- 

 GITUDINAL SECTION OF AN OLD 

 AND STOUT PORTION OF Cera- 

 mium rubrum, SHOWING CON- 

 TINUITY BETWEEN THE PROTO- 

 PLASMIC CONTENTS OF THE AXIAL 

 OR CENTRAL CELLS, a a, AT THEIR 



ENDS, AND LATERALLY WITH THE 



CORTICAL CELLS b, BY MEANS OF 

 PROTOPLASMIC THREADS. (After 

 Hick.) 



important, being the living substance. A protoplast which 

 has no cell-wall and contains no vacuole is still called a cell. 

 The term is again often applied to a cavity which contains 

 no protoplast, as in the case of old wood or cork. In such 

 cases a protoplast once occupied the cavity, but it has been 

 removed by death. These cells are consequently only the 

 skeletons of dead protoplasts, 



