116 PLANT PHYSIOLOGY 
168. Movements. Plant movements are of four 
kinds: (1) hygroscopic, (2) protoplasmic, (8) turgor, 
and (4) growth movements. The first is a strictly 
physical phenomenon of dead cells, the last three are 
functions of living cells or tissues. 
169. Hygroscopic Movements. Cell walls have a 
great power of imbibition of water, and when filled with 
water have a greater volume than when dry. In many 
plant organs certain cell walls have a greater power of 
imbibition than others, or in some cases certain tissues 
on one side prevent the organs from elongating or con- 
tracting on that side. The result in either case is that 
as the cell walls absorb water or give it up a curvature 
takes place. This may be asimple bending or may consist 
of twisting. Mostly the organs straighten out on becom- 
ing wet and curve or twist as they dry. In some cases the 
differences in the moisture content of the air are sufficient 
to produce movements. These movements are of value to 
the plant in opening reproductive organs (sporangia, seed 
capsules, etc.) or in enabling seeds to penetrate the ground 
(twisting of the long awn of porcupine grass, Stipa). 
170. In the case of the sporangia of the common ferns 
(Polypodiaceae), the cell lumen as well as the walls is 
filled with water. As the water evaporates through the 
cell wall, the cell 
contracts to compen- 
sate for the water 
lost. As the walls 
are thin and collap- 
sible on one side 
only, and thick but 
flexible on the 
others, the cell contracts more and more toward the thin 
side until the row of cells instead of being in a nearly 
Fia. 50.—Dispersal of fern spores. 
