346 LECTURE XV. 



transverse pressure upon the external tissues, but is stretched 

 radially by them, and to such an extent that it cannot keep 

 pace with their tangential growth, but ruptures, so that the 

 organs become hollow. 



Inasmuch as these tensions are the result of unequal tur- 

 gidity and growth and of the progress of histological differ- 

 entiation, we should expect to find that they are exhibited 

 in different degrees by different regions in a growing organ. 

 This has been determined experimentally by Kraus. He has 

 found, with regard to the longitudinal tension, that in the 

 youngest internode of a stem the pith does not alter in length 

 on isolation ; it was therefore not in a state of tension when 

 in the internode; but the external tissues, vascular, cortical, 

 and epidermal, shorten somewhat on isolation, and were there- 

 fore in a state of passive tension. In somewhat older inter- 

 nodes the pith is slightly compressed and the external tissues 

 stretched, the tension being greater the more external the 

 tissue; this stage corresponds to the numerical illustration 

 given above. In still older internodes the only perceptible 

 effect of the isolation of the tissues is the elongation of the 

 pith. These observations are to be explained thus; that in 

 the youngest internode the histological differentiation is rudi- 

 mentary and the external tissues are very extensible and 

 yield readily to the tension due to the expansion of the pith, 

 whereas in the older internodes histological differentiation 

 has proceeded so far that the vascular and epidermal tissues 

 are well-developed and their cell-walls have become thickened 

 and somewhat rigid, but they still yield to the expansion 

 of the pith; in the still older internodes the vascular and 

 epidermal tissues have become fully developed and are now 

 so rigid that they do not yield at all to the turgid pith, but 

 passively resist it. The transverse tension is distributed in 

 the manner described above for the longitudinal tension, but, 

 as already mentioned, the surface-growth of the organ may 

 eventually be so great as to cause a radial stretching of the 

 pith and even its rupture. The tensions are greatest in the 

 second of. the stages described above, when the external 

 tissues offer a certain elastic resistance to the expansion of 



