THE GROWTH OF THE CELL 261 



growth is confined to one small part of the wall, and this part may be either 

 terminal or, indeed, in any other definite region of the cell. In the first case 

 we speak of the growth as apical, and then the growth is distinctly unilateral in 

 relation to the full grown part of the wall ; the other case we term intercalary 

 where interpolation of a new region takes place between two zones which are 

 already fully developed. Examples of apical growth are to be found in roothairs, 

 pollen-tubes, fungal hyphae, &c. (Haberlandt, 1889, Reinhardt, 1892). Accord- 

 ing to Reinhardt growth is limited to the convex apex of the cell, to the minute 

 projecting region of the cylinder; further, growth is greatest at the extreme apex 

 and decreases gradually backwards. In Fig. 49 two stages in the growth of an apex 

 of a hypha are shown, and corresponding regions are indicated by dotted lines. 

 We see how much the surface cd has extended (to c'd'), and how little difference 

 there is between the surfaces ab and a'b'. The bestexampleof intercalary growth 

 is furnished by Oedogonium, where the intercalated region is marked ofi from 

 the older parts in the clearest possible manner. This example will be dealt 

 with in a greater detail later ; meanwhile reference may be made to Fig 51. 



Another case of intercalary growth is illustrated at Fig. 50, which shows 

 two stages in the development of stellate parenchyma. The originally closely 

 applied walls of two cells separate from each other at several points, and inter- 

 cellular spaces, a, appear between them. We may observe thereafter that the 

 cell-wall exhibits further growth practically only opposite the intercellular spaces, 

 while the regions where the two cells are in contact (in Fig. 50, /) scarcely grow 

 at all (Fig. 50, //). 



Surface growth of the cell-wall, like its primitive formation, takes place 

 only in the presence of protoplasm and nucleus, and, as a rule, growth in the wall 

 occurs only where the protoplasm is closely applied to it. This close application 

 of the protoplasm is firmly maintained by osmotic pressure, while at the same 

 time, and due to the same cause, the membrane is kept tense. Owing to the 

 fact that this osmotically-induced tension was observed in the majority of 

 growing cell-walls, and because a certain relation had been noted between 

 pressure and intensity of growth, it was for long considered that osmotic pressure 

 played a mechanical part in surface growth, and the phenomena which occur in 

 artificially formed cells were compared with those in a state of nature. Artificial 

 cells (Traube, 1867) m_ay be readily produced by taking a little gelatine to 

 which sugar has been added and allowing it to exude from the end of a glass 

 pipette submerged in a weak solution of tannin. A precipitation membrane 

 makes its appearance at once on the surface of the drop, a membrane whose 

 characters we have already studied (Lecture II). It is very permeable to 

 water, but quite impermeable both to tannin and gelatine. Under these con- 

 ditions an osmotic pressure develops inside this membrane which stretches it. 

 One of two things may now happen, either a simple extension of the 

 particles of the membrane by the intercalation of the gelatine solution between 

 those of the membrane, or fine cracks may appear permitting the exposure of 

 the gelatine solution, which, as soon as it comes in contact with the tannin, 

 develops at once a new precipitation membrane of gelatine tannate. Obviously 

 since the formation of new parts takes place quite regularly between the old parts 

 of the wall, the artificial cell will assume the form of a sphere of considerable size. 



Is there any likeness between the growth of the precipitation membrane 

 and that of the natural cell ? This question cannot be answered offhand. Of 

 course it is obvious that the membrane in the plant cell is not due to the pre- 

 cipitation of some product of the reaction of two fluids ; still osmotic pres- 

 sure might, for all that, play a mechanical part in surface growth. This has 

 indeed been accepted as true, and on this basis two theories have been advanced 

 to account for surface growth. According to one view, the cell- wall is simply 

 stretched mechanically by osmotic pressure and that too far beyond the limit 



