PHYSIOLOGICAL BOTANY. 26 I 



instance, each cell subdivides into two, and each of these 

 again into two more, and so on. As the internodes be- 

 come older, the growing process becomes still stronger, 

 because the expansion of the cells is then combined with 

 their increase in number. The growth decreases gradually 

 at last, because after the increase in the number of cells 

 formed has ceased, the expansion still continues to some 

 extent. 18. Hence three principal periods may be distin- 

 guished in the growth of the annual shoots of Dicotyle- 

 dons : 1st, that in which the intern ode still constitutes a 

 part of the bud, and where the increase in the number of 

 cells is only radial ; 2dly, that in which the internodes 

 increase both in length and breadth, and this again, a t 

 arising from the increase in the number of cells only, or 

 b y from simultaneous increase in the number and the 

 expansion of the cells, or lastly c, from the expansion of 

 the latter alone ; 3dly, that in which the growth in the 

 axial direction has ceased, but where the lateral expansion 

 still continues. 19. Since the longitudinal diameter of 

 the cells in those internodes in which the elongation has 

 ceased remains the same, the various different lengths of 

 the internodes must arise solely from the formation of a 

 larger number of horizontal layers. The author ascribes 

 the differences which are perceptible, when the effect of 

 season upon growth is considered, to this circumstance, 

 sometimes more, sometimes feAver of these layers being 

 developed. 20. In the cells of the pith and of the paren- 

 chyma of the bark of the youngest internodes, in which 

 the growth occurs slowly from the multiplication of cells, 

 we find a substance consisting of very small globules. 

 Very few of the cells are furnished with a nucleus (cyto- 

 blast) containing a corpuscle. On the other hand, we 

 find in many cells small groups, or simply rings, consist- 

 ing of these globules. On examining the adjacent older 

 internode, we recognise in a large number of cells, and in 

 the next internode (in which increase in the number and 

 expansion of the cells are simultaneously taking place) in 

 all the cells, perfectly-developed nuclei, which are quite 



