TURGOR MOVEMENTS 667 



on the lower side of the stem. It has been pointed out earh'er that growth 

 substances are known to accumulate in the lower half of horizontally placed 

 stem tips. It seems probable that similar accumulations of auxin may in- 

 fluence the nutational movements of the stem tips of twining plants. 



Turgor Movements. — In contrast to the growth movements which always 

 involve a permanent increase in the size or number of cells in the tissues con- 

 cerned there are many movements which result from reversible changes in cell 

 size. These are caused by variations, sometimes very rapid, of the turgor 

 pressure in the cells concerned. Because of their dependence upon changes in 

 turgor pressure, movements of this kind are known as turgor ?nove?nents. 



Turgor movements are responsible for the folding or rolling of the leaves 

 of many grasses during wilting, for the so-called "sleep movements" that 

 occur at night in the leaves of a number of species and for the sudden and 

 spectacular movements that occur in the "sensitive plant" {Mimosa pudica) 

 under the influence of various "stimuli." 



Many turgor movements are caused by variations in the turgor of spe- 

 cialized cells or organs. The folding and rolling movements of blue grass 

 leaves, for example, are produced by turgor changes in large, thin-walled 

 cells that are located on the upper leaf surface at the base of two grooves 

 that run parallel with the principal veins (Fig. 148). When turgor is high 

 the distention of these cells holds the leaf blade expanded and relatively flat 

 but when the turgor pressure of these cells decreases the pressure of the cells 

 on the opposite side of the leaf forces the leaf blade to fold. 



In the beach grass {Amniopfiila) such cells occur at the base of a number 

 of grooves in the upper surface of the leaf, and upon the loss of turgor of 

 these cells the leaf rolls up (Fig. 149). 



In some species of plants movements result from turgor changes in the 

 cells of pulvini. These structures are found in many species of the Legumin- 

 oseae. Pulvini are commonly located at the base of the petiole and at the 

 point of attachment of the leaf blade to the petiole. Externally they appear 

 as short, more or less swollen portions of the petiole. When pulvini are 

 present in compound leaves there is usually one at the point of attachment of 

 each leaflet to the petiole as well as one at the base of the petiole. A pulvinus 

 is composed of a compact mass of large, thin-walled cells which surround a 

 central vascular strand (Fig. 150). 



When all of the cells in a pulvinus are distended by their turgor pressure 

 the leaf is firmly supported. Movements result from sudden changes in 

 the turgor of the cells in one portion of the pulvinus while the turgor of 

 cells on the opposite side is maintained or even increased. The unequal 

 pressures thus arising on the two sides of the pulvinus cause the petiole to 



