GROWTH. 51 
sought the explanation of thin ears of wheat or defective 
hay crops. 
Phenomena associated with Growth and Activity.— 
Under this heading may be mentioned the various move- 
ments in the liquid (cell-sap), contained within the cells, 
and in the protoplasm, which are observed in living cells, 
especially in those in which the vital processes are most 
active. Here also may be mentioned the movements 
associated more or less directly with growth, and the 
influence of various agencies, such as of gravitation, 
light, temperature, etc., on plants and their several 
organs. These phenomena and these influences are more 
manifest during active growth ; and when they occur in 
living organs which have ceased their actual growth, 
they do not essentially differ, though they may do so in 
degree, and may also, to some extent, be modified in 
character. 
Movements as Dependent on Growth.—But a few 
years ago the notion of movement taking place in plants, 
other than that produced by the wind or other mechani- 
cal agency, was, if not entirely ignored, so little con- 
sidered that the immobility of plants was contrasted with 
the mobility of animals. We know now that even loco- 
motion is by no means an exclusive attribute of animals, 
but for our present purposes we need only refer to those 
movements more immediately connected with the growth. 
Movement of Protoplasm.—The protoplasm is a very 
mobile substance, and the cell-membrane is very elastic, 
while both, as has been shown, are permeable in various 
degrees by water, the consequence of which is that under 
favorable conditions the cells become turgid. As the 
degree of turgescence varies according to circumstances, 
tension being followed by flaccidity, and flaccidity over- 
