116 PLANT PHYSIOLOGY 



168. Movements. Plant movements are of four 

 kinds: (1) hygroscopic, (2) protoplasmic, (3) 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 iml^ibition 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 a simple 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 

 (Potypodiaceae), the cell lumen as well as the walls is 

 filled with water. As the water evaporates through the 



cell wall, the cell 



Ob, O, 



- o^ 



'^^^ <> 



contracts to compen- 

 ?=?^^I'VOo sate for the water 

 lost. As the walls 

 are thin and collap- 

 sible on one side 



Fig. 50.-Dispcrsal of fern spores. Ouly, 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 



