256 BOTANY part i 



when the protoplasm is fully siijiplied with water it is accumulated in vacuoles 

 (Fig. 57). As the vacuoles contain also organic and inorganic substances in 

 solution, they exert an attractive force and give rise to further absorption of 

 water. The sap of the vacuoles would, in turn, soon be diluted and its attractive 

 force diminished, M^ere it not that the regulative activity of the protoplasm soon 

 provides for a corresponding increase of the dissolved salts, so that the concentra- 

 tion and attractive force of the sap are continually being restored or even increased. 

 Thus, for example, by a change of glucose into oxalic acid the osmotic pressure can 

 be trebled. The separate vacuoles, thus enlarged, ultimately flow together into 

 one large sap-cavity in the middle of the cell. 



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 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 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 whether the growth of the cell wall is regarded as resulting from the inter- 

 polation 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 as to 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 tliat 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 

 coliesion 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 tlie fre(juent 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 honcycomb-likc system (Butschli). The new cellulose particles 

 would penetrate into the cell membrane, as particles of colouring matter nuiy be 

 introduced witli tlie iniliibitiou watei into a colloid organic substance (''"). 



