THE GROWTH OF THE CELL 265 



mesosporium there is a fluid from which a coagulable substance may be pre- 

 cipitated by alcohol (///, ger). The growing membrane is in contact with fluid 

 on both sides, from which it obtains the necessary nutrients and thus grows in 

 extent without being stretched by osmotic pressure. 



We are doubtless dealing here with an extreme case, since in general the 

 membrane grows superficially only so long as it remains in contact with proto- 

 plasm ; when the protoplasm is withdrawn from the wall by plasmolysis it 

 tends to excrete a new membrane on its outer surface. Why the sporogenesis 

 in Selaginella and Isoetes shows deviations from the normal has not as yet been 

 explained, but that this difference exists cannot be doubted after Fitting's 

 researches. The behaviour of the spore-walls in relation to osmotic pressure 

 is not so extreme, since it has been observed in other cases that a vigorous 

 surface growth can take place without an increase in turgidity, and that 

 growth in general may go on although turgor be reduced. Thus Pfeffer has 

 shown (1893) that the growth of the walls of the root-cells continues after osmotic 

 pressure has been more and more reduced by appropriate means, the cell-wall 

 finally becoming entirely, or almost entirely, contracted. In Pfeffer's ex- 

 periments this retraction was effected by a sheath of plaster of Paris, but 

 KoLKWiTZ (1896) showed that similar conditions may also occur in nature ; the 

 cells of the medulla of Helianthus, for example, exhibited growth at their ends 

 after they had been squeezed by the expansion of the vascular bundles. 



Such observations render it probable that the part played by turgor in 

 these and other cases is not merely mechanical. To effect plastic stretching 

 an expansive force is certainly necessary, and this force must be that of turgor ; 

 turgor is, however, quite unnecessary for growth by intussusception. This 

 pressure is indeed quite insignificant in comparison with molecular forces, such 

 as that of crystallization for example. Thus crystals of calcium oxalate may 

 originate and grow in the walls of certain cells (Pfeffer, 1892, 250 ; Muller, 

 1890), and these must have overcome the cohesion of the particles of the cell- 

 wall. Intussusception may therefore be best compared with phenomena of 

 this kind. 



One would think there ought to be no difficulty in settling experimentally 

 the part played by turgor in this process. If we place a turgid cell in an osmoti- 

 cally active solution the expansion of the wall is thereby reduced ; the stoppage 

 of growth which follows is, however, by no means a purely physical result of 

 the reduction of tension, but is due to a complicated stimulus action. Further, 

 we may increase the tension of the wall by reducing the osmotic value of the 

 external medium (e. g. in marine Algae), but in this case also the alteration acts 

 as a stimulus and growth ceases in consequence. In both cases, growth after 

 a time recommences, but in the interval a readjustment of osmotic pressure takes 

 place ; in short, it is a matter of great difficulty to determine experimentally 

 the relations between growth and turgidity. Theoretically we may say that 

 turgor, though it does not act directly by supplying the energy necessary for 

 growth, is nevertheless necessary for it. It may be considered ' as a condition 

 of growth, just as in an analogous manner a certain degree of heat may be so 

 regarded ' (Pfeffer, 1892, 219). Growth, however, does not appear to be 

 proportional to turgor pressure ; in many cases turgor appears to be regulated 

 by growth rather than growth by turgor (Copeland, 1896). 



Concomitantly with or after surface growth of the membrane growth in 

 thickness takes place. Very frequently this may be readily shown to be due to 

 the deposition of new lamellae, in other words, to apposition. If it continues 

 after surface growth ceases this apposition results in a lessening, and, finally, in 

 partial occlusion of the lumen of the cell ; hence this type of growth takes place in 

 opposition to the pressure of the cell contents. We are acquainted, however, 

 with other well-established observations which prove that a marked increase in 



