CHAPTER IV. 



EXCHANGE AND MOVEIIENTS OF GASES AND LIQUIDS. 



43. General physical nature of the plant. — The 



body of a plant is composed of a great number of units of living 

 matter, or protoplasts. From a physical point of view the 

 protoplast, or cell, may be regarded as a simple or compound 

 sac of protoplasm, or living matter, 

 which is automatic in its actions, and in 

 which chemical changes are constantly 

 in progress. The protoplasmic sac 

 encloses spaces containing solutions 

 known as vacuoles, and it is enclosed 

 by a wall or sac of cellulose. The 

 cellulose wall is not alive in the same 

 sense as the protoplasm, but its prop- 

 erties may be materially modified, and 

 its action is at all times under the 

 direct control of the protoplasm. A 

 constant interchange of material is in 

 progress between the vacuole and the 

 substance of the protoplasm, and be- 

 tween the vacuole and protoplast, and 

 the medium in contact with the cell 

 (Fig. 24) . In addition, the dead walls 



of mechanical cells form a small proportion of many of the sim- 

 pler plants of aquatic species and of the soft-bodied herbaceous 

 species. In species which live several seasons, a definite num- 

 ber of cells or a distinct layer perishes every season, and the 



Fig. 24. — Diagram of 

 structure of a cell, c, wall ; 

 u, lining layer of proto- 

 plasm ; ^, cytoplasm ; ^, 

 nucleus ; tr, large central 

 vacuole containing solu- 

 tions. After Hartig. 



