MOVEMENTS DUE TO TURGOR AND GROWTH 421 



If, however, the cell-wall be not of the same consistence all round, then an 

 alteration in osmotic pressure will always lead to an alteration in form. The best 

 known case of this kind is the movements of guard-cells, of which we have 

 already given an account. A glance at Fig. 8 (p. 39) will remind us that in the 

 guard-cell the convex side is thinner, and, therefore, more extensible than the 

 concave side. As osmotic pressure increases the curvature of the cell already 

 existing also increases, and it is easily seen that, by the appropriate distribution 

 of more resistant areas in the wall, a cylindrical cell may be made to exhibit not 

 only simple curvature but torsion and twining as well, such as are seen in Fig. 

 119 (p. 406). In nature, however, such torsions and twinings are due always 

 to phenomena of growth and scarcely to osmotic pressure. 



Movements arising from variations in turgidity occur much more fre- 

 quently in multicellular tissues than in single cells. Inasmuch as in these cases 

 the individual cells are osmotically unequally stretched there arise widespread 

 tissue tensions such as those referred to in Lect. XXIII (p. 297). Tissue tensions 

 were referable, as we found, to unequal degrees of growth in the separate com- 

 ponents of these tissues, but it is obvious that the only condition necessary for 

 tissue tension is the unequal efforts to elongate of the different parts, and, further, 

 that it is immaterial whether that elongation be effected by osmotic pressure, 

 growth, or some other factor. The example we took on that occasion was a stem 

 or similar structure, whose central region had greater powers of extension than 

 the periphery ; as a consequence, we found that the peripheral regions were 

 in a state of tension, while the medulla was in a state of compression, and that 

 the total length of the organ was the resultant of these opposing factors. 



So long as these antagonistic parts are distributed as they are in a normal 

 growing stem any alteration in the turgor conditions can only result in an altera- 

 tion in the length of the entire organ, and cannot induce any curvature, torsion, 

 &c. The significance of these tensions, which are of common occurrence, must 

 be purely mechanical ; for just as the single cell is rendered rigid by osmotic 

 pressure so a stem acquires rigidity from tissue tensions. 



In the typical stem, &c., we find that the tissues which contract are uni- 

 formly distributed all round the compressed central cylinder, but as soon as 

 that uniformity of distribution is interfered with curvatures at once take place. 

 Such disturbances are of frequent occurrence in nature, according as one 

 of the longitudinal halves of the organ under consideration gains or loses in 

 turgidity. Experimentally, it is perfectly easy to demonstrate the curvature 

 resulting from tissue tensions ; all one need do is to split a growing shoot-axis 

 longitudinally, when the medulla will thereby be enabled to extend itself and will 

 become convex, while the cortex in its efforts to contract will become concave. 



Movements due to variations in turgor are {re(\yiexit\y reversible if the factors 

 be also reversible, for the cell-walls are both extensible and elastic. Movements 

 of this kind are known as variation movements and stand out in contrast to 

 growth or nutation movements. These latter movements also start with stretch- 

 ing of the cell-walls, and hence may, in their earlier stages, be reversed by plas- 

 molysis, but after a short time the osmotically extended membrane under- 

 goes growth and its elongation and the movement itself become permanent. 

 Growth movements, like those due to turgor, may be rectilinear, curved, or 

 spiral, &c. It is unnecessary to consider this in further detail, for the analogy 

 with variation movements is complete. (As to growth itself, see Lect. XXI.) 

 A few remarks of a more general character may, however, be added here. 



In all movements, whether they be due to growth or turgor, a certain 

 amount of energy must be expended in the overcoming of external and internal 

 resistances. With internal resistances we are but slightly acquainted, but as to 

 the external, the elaborate experimental researches of Pfeffer (1893) have given 

 us very full particulars. These resistances may be very slight if the plant grows 



