254 BOTANY PAUT i 



cells of equal age in an organ go through this process of distension 

 at the same time, the phenomena of increase and decrease in the rate 

 of growth are apparent in the growth of the organ, and give rise to 

 GRAND PERIODS OF GROWTH. Minor periods, or fluctuations in the 

 rate of growth, occurring within the grand periods, are due to irregu- 

 larities in the swelling of the cells, occasioned by change of tempera- 

 ture, light, and other influences operative on growth, while the causes 

 of other abrupt changes in the rate of growth are still unknown ( 5 ). 



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. 178). Osmotic pressure seems 

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

 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 activity of the living protoplasm. 

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

 most distended cell wall ; on the other hand, active growth of the cell wall may 

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

 dence 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 import- 

 ance of the turgor tensipn is variously estimated, according to whether the growth 

 of the cell wall is regarded as resulting from the interpolation of new particles of 

 constructive material between the already existing particles of the cell wall 

 substance (INTUSSUSCEPTION) or to the plastic (i.e. inelastic, not resuming its 

 original position) expansion of the distended cell wall. In the latter case the 

 growing membrane would continually become thinner, and require to be strengthened 

 by the deposition of new layers upon it (APPOSITION). Both processes, which may 

 occur together, probably take part in the growth of cell walls. The necessity of a 

 certain amount of turgor, if growth is to result from plastic stretching, is self- 

 evident ; the stretching of the wall by the internal tension, though facilitating the 

 introduction of the new particles in growth by intussusception, is, however, not so 

 indispensable in this case. 



The assumption of a growth by intussusception is intimately related to the views 

 held on the finer (or so-called molecular) structure of organic substances. The power 

 of swelling in water, which may even lead to complete solution, exhibited by organic 

 substances, shows that the water of imbibition does not merely penetrate into pre- 

 existing capillary spaces, but makes a passage for itself by separating the solid 

 particles from one another. It is further evident that these particles must be of 

 minute (molecular) size. The intimate penetration of the water is the expression of 

 a powerful molecular attraction, which is capable of exerting an enormous force ; it 

 is rendered possible by the peculiar molecular construction of organic substances, the 

 cohesion of which is only gradually overcome by the water present in excess. The 

 arrangement of the particles has been pictured as resembling a network or a honey- 

 comb, while the frequent occurrence of double refraction as an optical property of 

 organic substances has been explained as due to the crystalline structure and definite 

 arrangement of the groups of molecules (micellae of NAEGELI) or to the relations of 

 tension in a colloidal honeycomb-like system (BiJTSCHLi). The new cellulose particles 



