GROWTH 171 



does not steadily rise and fall to and from the maximum, 

 but that there are " discontinuous" (stossweise) variations 

 apparently quite independent of the environment of the or- 

 ganism. It may be suggested that our analysis of growth, 

 according to which it consists in two distinct stages the one 

 fundamental, in which new protoplasm is formed, the other 

 evident, in which the cells expand may suggest a partial 

 explanation. Without a sufficient number of new cells, and 

 without a sufficient amount of new protoplasm, no expan- 

 sion can take place. Unless the two processes keep pace 

 with each other, the mensurable one will necessarily be ir- 

 regular. 



In this connection the fact already referred to ( pp.1 67, 168 ) , 

 that changes in volume may take place quite independently 

 of growth and because of turgor changes only, may be con- 

 sidered in somewhat more definite fashion. Kraus* pointed 

 out long ago, and has confirmed his observations made in 

 Europe by others in the tropics, that there are daily varia- 

 tions in the length and thickness of stems and branches, 

 leaves, buds, and fruits. "The diameter of a tree-trunk, for 

 instance, increases measurably till the early morning hours ; 

 it then decreases till nightfall, when it begins to increase 

 again." This is due to the variation in volume of the cor- 

 tical and other parenchyma cells caused by the difference in 

 the rate of transpiration at different hours of the day. Ab- 

 sorption by the root-hairs continuing at a rate much more 

 uniform than that of transpiration at night slightly 

 higher, by day slightly lower the turgor and the volume 

 of all living and sufficiently thin-walled cells will vary ac- 

 cordingly. This variation, wholly independent of all vital 

 functions, except those which govern the composition of 

 the cell-sap and the permeability of the protoplasm, con- 

 tinues in organs no longer growing, but may also, during 

 growth, contribute to the irregularities in the curve of 

 growth. 



The otherwise useful substances referred to above (p. 169) 



* Kraus, Gregor. /. c. II, find earlier in Die Wasservertheilung in der 

 Pflanze, 1881. nnd Die Gewebespannung des Stammes und ihre Folgen. 

 Botanische Zeitung, 1867. 



