PHYSIOLOGICAL BOTANY. 259 



retains the same proportion until the formation of wood 

 occurs. 6. The lignifying layers (vascular layers and liber) 

 do not begin to expand in a radial direction until the 

 fibrous cells have begun to acquire thickness ; but then 

 with a force which exceeds that with which the pith and 

 the parenchyma of the bark expands. 7. During this 

 period, the cavities of the cells and vessels expand uni- 

 formly, and this also still takes place after the thickening 

 of the fibrous cells has commenced. The increased space 

 which the vascular and prosenchymatous layers occupy in 

 the old segments, must be ascribed to this increase in 

 thickness, not therefore to deposition upon the inside of the 

 wall. The author says in a note, that the development of the 

 fruit in the Drupacese, and of the albumen in the seeds 

 of some Monocotyledons, shows, that the increased thick- 

 ness also arises from deposition upon the outside of the wall. 

 Must the expansion necessarily arise then from a thickening 

 of the walls ? The interstitial growth of cells and vessels, 

 which undoubtedly takes place in old stems, indicates an 

 expansion without any thickening. 8. The lateral expan- 

 sion of the cells composing the different layers, usually 

 takes place (at least in the pith, in the parenchyma of the 

 bark and of the epidermis) with the same force in all 

 directions. But this law is liable to exceptions according 

 to the activity of growth. 9. In the stems of those 

 plants in which no central canal is developed (Tilia and 

 Aristolochia) , the cells constituting the pith, the liber, 

 and the parenchyma of the bark, do not multiply peri- 

 pherally, but only in the direction of the longitudinal 

 axis. The peripheral multiplication is only perceived in 

 the epidermal and callenchymatous layers. 10. In the 

 plants just mentioned, neither the number of the vascular 

 bundles, nor, consequently, that of the vessels increases. 

 The diameter of the latter increases in proportion to the 

 expansion of the vascular layers. 11. In those plants, on 

 the contrary, in which a central canal is developed, the 

 cells of all the layers multiply peripherally, as is also the 

 case with the vessels. This multiplication, in the author's 



