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 

 fundcbinental, 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 thefactalreadyreferredto(pp. 167, 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, and earljei- in Die Wasservertheilung in der 

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

 Botanieche Zeitung, 1867. 



