172 PLANT PHYSIOLOGY 



are mainly water and certain salts not directly entering 

 into the construction of living protoplasm. Some of 

 the water forms an integral part of the protoplasmic 

 structure ( pp. 6-8 ) , but the greater part of it serves as the 

 vehicle of nutritious substances brought to the cell, and as 

 the solvent of all the soluble substances in the cell. As the 

 essential and invariable ingredient of cell-sap, it is the mate- 

 rial which maintains the second or evident stage of growth. 

 The volume of the cell depends upon the water and upon 

 the compounds dissolved in it. The composition of the cell- 

 sap is regulated by the living protoplasm which adds to 

 or takes from it soluble compounds of diverse sorts, — 

 assimilable and excreted matters, such as the sugars and 

 organic acids respectively. Besides these, it is claimed that 

 the cell owes its turgescence to certain soil-constituents, 

 especially the salts of potassium. According to Copeland, * 

 the degree of turgescence in ordinary roots, stems, and 

 leaves is only slightly dependent upon food-manufacture, 

 and is mainly due to a substance or to substances which 

 cannot be used to keep the plant from starving. Copeland 

 concludes from his experiments on the effects of light and 

 darkness, heat and cold, that the rate of growth has much 

 more effect upon the turgor of the growing part than vice 

 versa. 



The fundamental stage of growth, consisting in the for- 

 mation of new protoplasm, implies the intussusception or 

 interpolation of new particles between the older parts of the 

 structure, or the application or apposition of new particles 

 upon the older, or both of these processes. In either some 

 force must be exerted. If the cell is distended while the new 

 particles are being formed and placed, the introduction of 

 new particles between the older will be proportionally easier. 

 So the turgescence of the cell, tending to keep all parts 

 stretched, may contribute to such growth. But Copeland 

 had in mind evident growth, increase in volume, rather 

 than the formation of new protoplasm. The turgor of 

 the cell may be sufficient to stretch it, to increase its vol- 



* Copeland, E. B. Tiber den Einfluss von Licht und Temperatur auf 

 den Turgor. Inaug. Diss., Halle, 1896. 



