sect, ii PHYSIOLOGY 231 



larities in the swelling of the cells, occasioned by change of temperature, 

 light and other influences operative on growth. 



The large amount of water absorbed by the growing organ in the process of 

 elongation does not lessen its rigidity, but, on the contrary, it is to the turgor 

 thus maintained that the rigidity is due (cf. p. 165). Osmotic pressure seems 

 also to take an important part in the growth of the cell wall itself. Cells in which 

 the turgor is destroyed by a decrease in the water-supply exhibit no growth of 

 their cell walls ; it is thus evident that the distension of the cell walls is physically 

 essential for their surface-growth. This distension is in itself, however, by no 

 means the cause of their growth ; the internal physiological conditions of the 

 growth of the cell walls are dependent upon the vital activity of the living proto- 

 plasm. Without the concurrent action of the protoplasm, there is no growth in 

 even the most distended cell wall ; on the contrary, active growth of the cell wall 

 may take place with the existence of only a small degree of turgor tension. A 

 CORRESPONDENCE BETWEEN THE TURGOR TENSION OF THE CELL WALLS AND THE 

 AMOUNT OF GROWTH CANNOT, UNDER THESE CONDITIONS, BE EXPECTED, nor Can, on 

 the other hand, the conclusion be drawn that turgor tension is inoperative in the 

 processes of growth. The importance of the turgor tension is variously estimated, 

 according to the conception of the manner in which the growth in substance of the 

 cell walls takes place. There have been for some time two conflicting theories in 

 regard to this. According to one, the growth of the cell wall is due to the inter- 

 polation of new particles of constructive material between the already existing 

 particles of the cell -wall substance (intussusception); in the other theory, the 

 assumption of the interpolation of new particles is disputed, and growth in surface 

 is attributed to the plastic (inelastic, not resuming its original position) expansion 

 of the distended cell wall. As in this case the growing membrane would continu- 

 ally become thinner, its growth in thickness results from the repeated deposition 

 of new layers (apposition) of substance on the internal surface of the original wall. 

 It is, however, a question of purely theoretical interest, by which of the methods 

 the growth of the cell membrane is effected in particular cases. While, in general, 

 neither of these views is inconsistent with the external phenomena of growth, in 

 some special cases intussusception, and in others apposition, seems to offer the 

 more satisfactory explanation. It is, in fact, not improbable that the growth of 

 the cell walls is due to both processes. It is evident that at least some degree 

 of turgor tension is necessary for the existence of this form of expansion. To support 

 the theory of intussusception it has also been found necessary to suppose that the 

 new particles are not interpolated until the spaces between the particles of the 

 cell- wall substance has been enlarged by the distension of the wall itself. 



The process of elongation has so far been considered only in 

 relation to the single cell, preparatory to the consideration of the 

 phenomena presented by the growth of multicellular organs. 



The operations of growth in plant organs proceed very slowly ; so 

 slowly as to be, in general, imperceptible. The stamens, however, of 

 many Gramineae grow so rapidly that their elongation is evident, even 

 to the naked eye. An increase in length of 1*8 mm. a minute has 

 been observed in the stamens of Triticum (Wheat). This approximately 

 corresponds to the rate of movement of the minute-hand of a watch. 

 In comparison with it, the next known most rapidly growing organ 



