212 THE LIFE OF THE PLANT 



acquaintance a little while ago with phenomena of 

 longitudinal tension, i.e. of tension depending on the 

 unequal elongation of different tissues in a stem. 

 Similar inequalities of growth can and do also take 

 place in the transverse direction. The bark is con- 

 tinually compressing the wood which grows more 

 quickly than itself, and in its turn it is continually 

 stretched and strained by the latter. This is proved by 

 the longitudinal fissures that generally appear in the 

 bark owing to the internal pressure of the wood. It is 

 not difficult to see that the smoothest bark is usually 

 in a taut condition, and therefore presses on the wood. 

 If we make a longitudinal cut with a knife, we shall 

 see that the wound will gape ; or, better still, if we cut 

 off a ring of bark and replace it immediately (without 

 giving it time to dry) in the same place, we shall notice 

 that the edges will no longer fit and no effort will bring 

 them together again. This means that the bark presses 

 like an iron ring on the growing wood, and the more the 

 latter develops the stronger is the pressure it has to 

 overcome. Evidently this pressure will continually 

 increase, and in autumn reach its maximum. Owing to 

 this increasing pressure the cells of the wood will 

 become increasingly flat. The truth of these statements 

 is fully supported by experiment. If the pressure on 

 the wood is artificially increased at the beginning of 

 spring by an iron ring being girt round it, cells similar 

 to the autumn ones will appear at those places through- 

 out the year ; whereas, on the contrary, if the pressure 

 of the bark is weakened in summer and autumn by 

 means of longitudinal cuts, we shall find in those places 

 throughout the year cells similar to those found in the 

 spring. Such, therefore, are the changes of form in the 

 cells, which can be caused simply by mechanical means.^ 



• Another curious effect of a. purely mechanical influence upon the 

 growth of tissues has been demonstrated. The movements of the trunks 

 of trees caused by the wind (as was shown at the beginning of the 



