THE CELL. 29 



a short distance below this. The growing surface consists approxi- 

 mately of the hemispherical apex, with an additional cylindrical 

 portion equal in height to the radius of the hemisphere. Under 

 certain conditions rupturing of the hyphae with extrusion of plasm 

 takes place. This rupturing, however, does not take place at the 

 point of greatest curvature and maximum growth in the apical area, 

 but usually at that point below the tip where the cell-wall has ac- 

 quired its greatest thickness, hence at the base of the growing part. 

 The apposition theory, which teaches that inner lamellae or layers are 

 formed by apposition and outer ones are ruptured by expansion, 

 would only permit rupture of the cell-membrane at the place of 

 maximum growth, that is, in the region of maximum curvature, 

 which is not the case. Rupturing takes place at the points of greatest 

 tangential tension because of the hydrostatic pressure, not at the 

 points of supposed greatest expansion. According to Reinhardfs 

 assumption apical growth of the cell takes place in such a manner 

 that given points lying nearest the axis at the apical area form tra- 

 jectural curves outward with the advance (growth) of the apex. 



The theory of intussusception is based upon deeper and more 

 far-reaching considerations underlying the sphere of molecular 

 physics as opposed to the more crude theory of apposition. As al- 

 ready indicated, the latter theory presupposes considerable mechani- 

 cal expansion for the surface growth of cell-membranes. Let us 

 follow Nageli's ' arguments more closely. The cylindrical cells of 

 the alga Spirogyra grow considerably in length while retaining 

 nearly the same thickness. Intussusception requires only an infi- 

 nitely small reduction in the cohesion of molecules in the longitu- 

 dinal direction to make it possible for new molecules to be 

 intercalated. Tension is thus equalized, to be again disturbed by 

 the same infinitely small difference. 



No difference can ~be demonstrated in molecular cohesion in a 

 longitudinal or transverse direction of the cell-wall. According to 

 the apposition theory the difference in cohesion in the two direc- 

 tions must be very great, because it presupposes that layers must be 

 torn apart in a longitudinal direction. The intussusception theory 

 also postulates a very slight diminution in tension in a longitudinal 

 direction. 



Before dismissing this difficult subject it should be noted that 



1 Starkekorner, page 279 et seq. 



