8io 
MECHANICS OF GROWTH, 
the rapidity of the growth of cells is in proportion to the thinness and therefore 
the extensibility of their walls. The growth in thickness of the cell wall usually 
begins when the increase of the cell in volume begins to diminish or has altogether 
ceased. 
If then the distension of the cell-wall caused by turgidity is the origin of its 
superficial growth, something similar must also occur when the cell-wall is stretched 
in some other way by external forces, the turgidity being less. This is the case 
with the epidermis and cortex of shoots as a result of the tension of the tissues. 
Since in long internodes and leaves these cells usually grow principally in the longi- 
tudinal direction, while in broad leaf-blades they assume the form of polygonal 
plates, this may be referred in the first case partly to the traction to which they are 
subject being chiefly in the longitudinal direction, in the second case to its being in 
all directions parallel with the surface ^ It has already been stated that the cells of 
the primary cortex of shoots which are increasing rapidly in thickness are not merely 
stretched but also grow rapidly in the tangential direction 
2. Pressure from without on the cell-wall which is distended by turgidity occurs 
in a very simple form when the apices of growing cells come into contact with solid 
bodies ; as the root-hairs of land-plants with the particles of the soil ^ The very thin 
and extensible cell-walls are in close contact wnih. 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 dis- 
tension. The reverse takes place when the external pressure on the cell-wall is 
removed. A very simple instance of this is afforded by the formation of the so- 
called ' Tüllen ' in vessels *. They are formed where the thin non-lignified wall 
of a cell of the wood-parenchyma, still capable of growth, adjoins the 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 
^ For further details on the possible influence of tension on the formation of stomata, see 
Pfitzer, Jahrb. für wiss. Bot. vol. VII. p. 542. 
^ On the connection of the radial and peripheral arrangement of rows of cells in a transverse 
segment with the increase in diameter, see the lucid description of Nägeli in his Dickenwachsthum 
des Stengels bei den Sapindaceen, Munich 1864, p. 13 seq.: [also Detlefsen {loc. cit.).'] 
3 Sachs, Experimental-Physiologic, p. i85. 
See Book I. p. 26 [and references in foot-note. These growths are frequently termed ' tyloses,'] 
