MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 73 1 



and extensible cell-walls are in close contact with the irregular surface of the par- 

 ticles, just as when an elastic bladder filled with water is pressed externally by an 

 angular body, only that they retain, after the pressure is removed, the form which has 

 thus been given them, evidently in consequence of the intercalation of fresh particles 

 of solid matter which perpetuates the form at first acquired only by distension. 

 The reverse takes place when the external pressure on the cell-wall is removed. 

 Avery simple instance of this is afforded by the formation of the so-called 'Tiillen' in 

 vessels ^ These appearances are produced where the thin non-lignified wall of a 

 cell of the wood-parenchyma, sdll capable of growth, adjoins the bordered pits of 

 a vessel. The portion of wall which is stretched over the opening is forced through 

 it by the pressure of the sap of the cell and swells out in the form of a papilla into 

 the cavity of the vessel. As long as the vessel contained sap and was in a turgid 

 state, its turgidity was in equilibrium with that of the adjoining cell ; but as soon as 

 the cell-sap of the vessel was absorbed, the portion of cell-wall which covers the 

 bordered pit was subject to pressure on one side only, and was therefore forced in 

 the opposite direction. These phenomena can be produced artificially by the 

 removal of the pressure to which the cells are subject from the adjacent tissues ; 

 thus, for example, the cambium swells up on the cut surface of woody branches 

 when placed in moist sand or air, in the form of a cushion between the bark and 

 the wood. This ' Callus,' as it is termed, results from the growth of the uninjured 

 cambial and adjoining cortical cells next the cut, where their growth was previously 

 prevented by the cells which have now been removed. When once projecting 

 beyond the cut, they grow more rapidly than before in a lateral direction in conse- 

 quence of the turgidity, and become divided by transverse and longitudinal walls 2. 



The further development of such a callus where branches have been cut off leads 

 to the well-known overgrowth on the stumps. In internodes of seedlings of Phar 

 seolus which had accidentally become hollow, 1 found the medullary cells which 

 surrounded the cavity to have grown into it in the form of spherical or club-shaped 

 papillae ; divisions ensued, and nuclei were formed in the cells thus produced. The 

 medullary cells which exhibited this active growth on the free surfaces of their walls 

 would have retained their polyhedral form had the pith remained solid, because every 

 surface of the cell-wall would have been exposed to the pressure of the two adjoin- 

 ing cells ; but in consequence of the formation of the hollow, the pressure was 

 removed on one side, and the turgidity, being no longer neutralised, caused the cell- 

 wall to swell out, and induced in it an active superficial growth^ These phenomena 

 and others of the same kind show that it is often sufficient merely to remove the 

 pressure to \yhich tissues or individual cells are subject in order to bring about an 

 active growth of the free surfaces of their cell-walls. The first cause at least of 

 the new growth is the distension of the free surfaces of the cell-walls in consequence 

 of the turgidity of their cells which was previously neutralised by that of the adjoin- 

 ing cells. But that a very small pressure from without is sufficient to prevent the 

 growth of softer tissues at the points of contact is seen in the case of many large 



' See Book I, p. 27 [and references in foot-note]. 



^ Further details on this point will be given in a yet unpublished memoir by Prantl. 



^ Prantl succeeded in artificially inducing similar phenomena in the tubers of Dahlia. 



