392 H. von Mohl on the Camhium-lmjer 



B. If the formative process advances regularly from below 

 upwards, a definite plane of the base ceasing to form cells, a 

 cylindrical ascending axis is formed. This process occurs in 

 stems with elongated internodes. 



c. If the process of cell-formation ceases at particular parts 

 of the periphery earlier than at others, the result is an axis with 

 pi'ojecting ridges, trigonous, &c. 



D. If the cell-formation lasts longer at the circumference than 

 in the middle, and the terminal bud is of the usual conical form, 

 the cell-formation occurs, not in the whole cone, but only in a 

 superficial region, forming a kind of conical cap or mantle on 

 the cone [Kegelmantel), so that the whole free surface of the cone 

 contains the younger cells, the central part of the cone the older. 

 Here the whole axis of the cone ordinarily rises cylindrically 

 upwards ; but not by similar superposed disks (as in b), but by 

 superposed hollow cones {Keyelmantel). Each new internode is 

 a hollow cone of this kind, and therefore cannot be cut off by a 

 section perpendicular to the axis, but only by a cut following the 

 surface of the cone. If the process of cell- formation lasts some- 

 what longer in the later internodes than in the earlier, a more 

 elongated hollow cone is formed, which consequently projects 

 over the base of its predecessor, which would otherwise be free, 

 and the new internode becomes broader in proportion to the 

 older (in Melocactus, Zea, &c.). 



E. If the cell-formation ceases earlier at the margin than in 

 the middle, and the new cells formed in the middle arrange 

 themselves successively in planes, the margin must rise up, the 

 middle becoming gradually developed into a hollow form, just 

 as a disk of lead becomes concave when it is hammered in the 

 middle and not at the edges. In this way is produced the 

 funnel-shaped end of the stem of Echinocactus, the calyx of 

 Rosa, &c. 



In another place (ii. p. 147) Schleiden gives the following 

 explanation of the variation of the internal organization of stems, 

 in which he sets out from the idea that, as the cellular tissue is 

 formed, a portion of it is always changed into vascular bundles ; 

 consequently the direction of the vascular bundles depends 

 wholly upon the direction of the formative energy. On this 

 account, in long-jointed stems, where the cell-development takes 

 place from below upwards, as it were, in horizontal disks, the 

 vascular bundles are straight and tolerably parallel to the axis of 

 the stem ; where, on the other hand, one hollow cone is super- 

 posed on another in the terminal shoot, the vascular bundles 

 take, in their first formation, a course from the base of the cone 

 to its summit, therefore from the circumference of the internode 

 to its axis; and subsequently, when new internodes are super- 



