GROWTH 167 



crease in volume due to the absorption of water is genuine 

 growth, and the second stage in growth, the one which I 

 have designated as evident growth, consists mainly if not 

 wholly in the absorption of water. But as the water-con- 

 tent of any cell or tissue or organ is subject to fluctua- 

 tion, and as the turgor and, consequently, the volume 

 also fluctuate with the water-content, it is difBcult to tell 

 when growth ceases and turgor-swelling begins. Both are 

 vital processes in the sense that they depend upon the 

 physical and chemical conditions established by the living 

 protoplasm, but they are distinct processes, fluctuations in 

 size due to changes in turgor taking place even long after 

 true growth has ceased to be possible (see pp. 167, 168). 

 Whether growth depends upon turgor or vice versa is still 

 to be conclusively shown by experiment. 



That evident growth consists mainly in the increase of the 

 water-content of the growing part has long been known to 

 be the case in plants, though only recently properly em- 

 phasized for animals. * A longitudinal section through the 

 tip of a growing stem or root, or a cross-section through 

 the stem of a dicotyledonous plant during its period of 

 growth in diameter, will show at least three distinguishable 

 regions. These are indicated in the accompanying figures ( pp. 

 168, 169 ) of a longitudinally sectioned root of AzoUa. In the 

 diagramatic Figure 7 the three regions of cell-formation ( 1 ) , 

 cell-growth (2), and cell-differentiation (3), are indicated. 

 These are shown in detail in the figures 8, 9, 10. Figure 8 

 (corresponding to 1 in 7) represents the tip of the root 

 with its cap {Cap), dermatogen and epidermis (Ep.), 

 cortex (Cor.), and central cylinder (c. c.), aU of which 

 come directly or indirectly from the division of the large 

 apical cell. The meristematic and embryonic cells are full of 

 dense protoplasm. Figure 9, taken from further up in the 

 same root, corresponds to region 2 in figure 7, and shows 

 that the increase in size of the cells of the different layers is 

 accompanied by a great increase in the volume of the cell- 



* Davenport, C. B. The role of water in growth. Proe. Boston Soc. 

 Nat. History, vol. 28. 1897. Experimental Morphology, Part II., 1899, 

 and the literature there cited. 



