260 PHYSIOLOGICAL BOTANY. 



opinion, causes the absorption of the fluids in the internal 

 cells, and their desiccation, which explains the origin of 

 the inner cavity. 12. When canals containing gum occur 

 in the pith or in the parenchyma of the bark (Tilia), they 

 increase but little in diameter during their growth, but 

 they multiply, and when the elongation is completed, they 

 again diminish and become thickened. They already 

 exist at the very earliest period. 13. In those stems in 

 which a central canal is not formed, the increase in breadth 

 depends upon the radial expansion of the cells, excepting 

 in the layers of the callenchyma and the pith. In those 

 stems in which a central canal occurs, the part which 

 the multiplication and expansion of the cells takes in its 

 formation, is variable. 14. This is also the case in the 

 longitudinal growth. 15. The longitudinal multiplication 

 of the cells, as also their expansion, takes place simulta- 

 neously at all points of the internode, but in those inter- 

 nodes which are still elongating, the cells of the pith, of 

 the parenchyma, of the bark, and of the epidermis at the 

 apex of the internode, are shorter than those at its base, 

 and the latter again are shorter than those at the apex of 

 the adjacent older internode. When the expansion of 

 the cells at the base has ceased, that of the cells at the 

 apex still proceeds for some time. 16. The smallest cells 

 multiply most, hence the epidermal cells more than those 

 of the parenchyma of the bark, and the latter more than 

 those of the pith, but no definite proportion is observable. 

 17. Whilst the internode is still very young, the growth 

 takes place principally by the multiplication of the cells 

 only. When the internodes of a plant, after they have 

 acquired their full length, are of slightly different lengths 

 (Tilia, Humulus, and Aristolochia) , the numbers of the 

 cells of the pith and bark, in the younger internodes, form 

 a geometrical progression. Moreover, the younger the 

 internodes are, the less they grow, and when the growth 

 is accelerated with advancing age, it observes a geome- 

 trical progression. All this shows that the multiplication 

 of cells takes place in geometrical progression. For 



