398 H. von Mohl on the Cambium-layer 



its axis, and subsequently, when new internodes are superadded, 

 are developed onward through the new cambium-cones, to reach 

 the buds and leaves. It is certainly conceivable that the vascular 

 bundles might arrive at the middle of the stem already at the 

 end of the first internode, and in the second internode run into 

 the leaf, although no such stem has yet presented itself to me ; 

 but that would not alter the matter at all, and in such a bud 

 there would be just as little correspondence of a special cambium- 

 cone to each internode, as in a plant in which each vascular 

 bundle runs through a dozen internodes. 



Completely erroneous also is the idea that in short-jointed 

 stems the internodes have a conical form corresponding to the 

 form of the cambium-mantle, and stick into one another like 

 funnels, so that they cannot be separated from one another by a 

 horizontal section. A longitudinal section through the apex of 

 a distinctly-jointed stem, for instance of Arundo Donax, shows 

 that the internodes are not attenuated upwards, but that they 

 are either separated from one another by horizontal planes, or 

 are depressed downwards, so as to be concave. 



This last condition leads me to the consideration of the form 

 of stem spoken of by Schleiden under e, in which the bud is 

 excavated in the middle. This not unfrequent depression of the 

 point of the axis he believes to arise from the cell-formation 

 ceasing sooner at the margins than in the middle, and that con- 

 sequently the middle of the internodes assumes a hollow form, 

 like a piece of metal plate when beaten out in the middle. This 

 might happen if the internodes, like the metal plate, were free 

 underneath; but as its under surface is continuous with the 

 already more developed and more solid tissue of the subjacent 

 internodes, a predominant development of cells in its centre 

 would, to keep to Schleiden's comparison, no more render it 

 hollow, than hammering a plate of metal, soldered to a block, 

 would form it into a bowl. Cell-development predominant in 

 the middle of the internode could only cause its expansion up- 

 wards, thus producing the opposite of what Schleiden expected. 

 It is easily seen that such expansion of the punctum vegetationis 

 is a consequence of just the opposite condition, — that the deve- 

 lopment of cells at the circumference is in excess, and the cells 

 in the middle are further behind in their development — this 

 stage being followed by a second period of growth, in which the 

 cells of the centre extend longitudinally, whereby the meniscus 

 form of the internodes is converted into a discoid, and the in- 

 wardly-curved surface of the stem is curved outwards and con- 

 verted into a cylinder, as has been demonstrated most con- 

 vincingly by Hofmeister in the depressed summits of the stems 

 of Ferns. 



