GROWTH. 349 



certain areas of the cell-wall to become less dense, more 

 capable of imbibition (p. 15), and then also more extensible, 

 than the rest. No adequate explanation can at present be 

 given of the nature of this action of the protoplasm, but there 

 can be no doubt that it actually takes place. As an illustration 

 of its importance in relation to change of form, Strasburger 

 mentions that when a lateral branch is to be developed upon 

 a somewhat old cell of Cladophora, an area of the cell-wall, 

 at the point where the branch is subsequently borne, swells 

 up by imbibition, becomes more extensible than the rest of 

 the wall, and, under the pressure of turgidity, is forced 

 outwards so as to form a protuberance. The further growth 

 of this protuberance is effected by the same means : the wall 

 remains extensible at the apex, and thus the protuberance 

 elongates until the limit of growth is reached. It is then, 

 in any case, by the protoplasm that the distribution of ex- 

 tensibility in the growing cell-wall is determined, and it is 

 therefore upon the protoplasm that the ultimate form of the 

 cell depends. 



We will now pass to the second of the two points raised 

 above, namely, the effect of the tension of the tissues upon 

 the form ultimately assumed by the cells of which they con- 

 sist. It is obvious that when a number of cells are closely 

 coherent, as in a growing point, the pressure which they 

 mutually exert must to some extent modify the natural 

 growth of the individual cells, and must therefore considerably 

 affect the ultimate form of the cells. Examples of this are 

 constantly presenting themselves in the study of the histology 

 of plants. The most striking of these are produced by the 

 transverse tension, especially that which is set up in conse- 

 quence of the secondary growth in thickness of stems and 

 roots. Still the longitudinal tension is not without its effect 

 in this respect, though it is not so readily demonstrable. 

 There can be no doubt, for instance, that the elongated form 

 of the cells of the fibrous and vascular tissues is due in great 

 part to the stretching to which they were subjected, as de- 

 scribed above, by the turgid pith whilst they were still grow- 

 ing : and conversely, the resistance of the peripheral tissues 



