762 PART IV. THE PHYSIOLOGY OF PLANTS. 



unlignified walls, the lignified tissue being reduced as much as 

 possible : similarly, in the elongating region of growing-members 

 the cell-walls are thin and unlignified. But whilst the movements 

 of variation (p. 737) are the result of a sudden loss of turgidity, 

 which is either spontaneous or the effect of stimulation, the move- 

 ment of growth depends upon the maintenance of turgidity, and 

 the variations in the rate of growth (see p. 738) are the expression 

 of variations in the degree of turgidifcy. 



Brief allusion may here be made to the controversy as to the mechanics of 

 growth of the cell-wall. The two conflicting theories are (1) the theory of 

 intussusception, according to which the growth of the cell-wall is due to the 

 intercalation of new particles (micella;) of solid suhstance between the already 

 existing particles of the wall ; and (2) the theory of apposition, according to 

 which the growth of the cell-wall is due to the repeated deposition of layers of 

 substance on the internal surface of the original wall. Neither of these theories 

 gives a completely satisfactory explanation of all the phenomena of growth of 

 the cell-wall, nor are they mutually exclusive. It seems that the growth in 

 surface of the cell- wall can be most satisfactorily explained on the intussus- 

 ception-theory, and the growth in thickness on the apposition-theory. Both 

 theories, as generally stated, seem to involve the assumption that the growth in 

 surface of the cell-wall is dependent upon the turgidity of the cell : but this 

 assumption is not necessary to the intussusception-theory, and it is probably 

 even inadmissible. 



The following instances will serve to illustrate the foregoing 

 considerations. 



A simple case is offered by the induced movement of the stamens 

 of the Cynareae (p. 692). When at rest, the cells of the filaments 

 are expanded in the direction of their length, and are turgid ; on 

 stimulation, the cells suddenly shorten and become flaccid, having 

 lost a portion of their cell-sap. The expanded state is regained by 

 the gradual expansion of the cells, turgidity being restored by the 

 absorption of water. 



In the foregoing case, all the cells of the motile portion are 

 affected ; but in many cases some only of the cells are affected. 

 Thus, in the case of the leaf of the Sensitive Plant, the primary 

 petiole, when at rest, stands out nearly at right angles to the stem 

 (Fig. 472, p. 690) : on stimulation, it sinks downwards so as to 

 form an acute angle with the internode below its insertion. The 

 mechanism is this : when at rest, the cells of the pulvinus are all 

 turgid, and they support the petiole in its normal position: on 

 stimulation, the cells of the lower portion of the pulvinus lose 

 their turgidity, water escaping from them into the intercellular 



