39 



plants as Dahlia and Althaea the intercellular spaces in the pith 

 at the time the casts were applied were very small and the cells still 

 vigorous, and consequently the movement toward the centre was not 

 to be distinguished. In the 3rd group of plants noted, the move- 

 ment toward the centre takes place or not according to the inactive 

 or active condition of the pith respectively and the absence or the 

 presence there of large intercellular spaces. Other things being 

 equal, the younger the plant the more active in extension will the 

 pith be. 



Though the movement of tissues toward the centre in the cases 

 mentioned generally indicates that the energy of the cortex is greater 

 than that of the pith, it does not indicate that the cells of the young 

 bundles are less energetical than those of the cortex. The bundles 

 and the meristem between them also bear a part in closing up the 

 intercellular spaces, but the amount of tissue there and the extension 

 of cells are, compared with the cortex, so small that the effect is much 

 less apparent. 



A very striking effect of enclosing an internode in gypsum be- 

 fore primary growth has ended is the smaller definitive size attained 

 by the parenchyma cells of the fundamental tissue. Such cells can 

 from the nature of the case increase in length by gliding growth 

 only, and this is shown by observation to be small. They can widen 

 only to the extent of filling the intercellular spaces; and the room 

 thus afforded varies with different species and with different ages in 

 the same species. An internode for instance that has nearly com- 

 pleted its primary grow r th will have larger cells and larger intercel- 

 lular spaces than one in which primary extension is farther from its 

 termination. 



It might be expected that the plant when it cannot produce so 

 great an extent of cell-membrane as normal would make thicker 

 membranes, and Wortmann l describes cases in which he has obtained 

 this result. This Author wound the young stems of several plants 

 with twine, thus stopping, he says, longitudinal extension, and radial 

 also except for the growth made between the coils of the imperfectly 

 fitting cord. An examination of structure after a few days' growth 

 show r ed in the cortex of the constricted part smaller cells with 

 thicker membranes than normal. 



Pfeffer however, in the manuscript referred to, states that in the 

 root-tips that had been encased in gypsum the cell-walls became 

 no thicker. The 15 species of plants including scores of individuals 



Wortmann, Beitrage zur Physiologie des Wachsthums. Bot. Zeit. 1889, p. 286. 



