16 



VEGETABLE PHYSIOLOGY 



In a few cases, as in certain seaweeds, and in the sieve- 

 tubes of the flowering plants, the connecting strands are 

 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 



Fio. 17. CONTINUITY OF THE PROTOPLASM OF 

 CONTIGUOUS CELLS OF THE ENDOSPERM 

 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 Ceramium ru- 

 brum, SHOWING CONTINUITY BE- 

 TWEEN THE PROTOPLASMIC CON- 

 TENTS OF THE AXIAL OR CENTRAL 

 CELLS, a, a, AT THEIR ENDS, AND 



LATERALLY WITH THE CORTICAL 



CELLS b, BY MEANS OF PROTO- 

 PLASMIC THREADS. (After Hick.) 



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

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

 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 or cavities are consequently 

 only the skeletons of dead protoplasts. 



