M. Mohl on the Growth of Cell-Membrane. 153 



which we meet with in the fully developed condition of the in- 

 ternode is not to be ascribed to the occurrence of a new forma- 

 tion of cells during the summer, it may not be superfluous to 

 direct our attention to a second relation, which decides the fact 

 in the most indubitable manner. In the examination of young 

 shoots of dicotyledonous trees, e. g. of oaks, poplars, robinias, 

 &c, we find, without exception, that their vascular bundles run 

 downward from the base of the leaf through several internodes 

 in a parallel direction without entering into any lateral connexion 

 with each other *. The medullary ray lying between two vas- 

 cular bundles has also a length equal at least to an internode. 

 The same relations are met with also, unchanged, in full-grown 

 twigs in the inner part of the wood, in the so-called corona, 

 which corresponds to the young vascular bundles; the larger 

 and more externally situated portion of the wood, on the con- 

 trary, exhibits an essentially different mechanical arrangement 

 of its constituent elements. There are, in particular, no longer 

 any separate vascular bundles to be distinguished, but the whole 

 woody mass forms a continuous cylinder, the fibrous bundles of 

 which exhibit not a straight but a serpentine course, and have 

 grown together at certain distances, so as to form a network of 

 narrow and not very long meshes, which are filled up by the 

 medullary rays. In the very young internode there is not the 

 slightest trace of all this reticulated layer, which at the end of the 

 year forms the greater proportion of the body of the wood ; in 

 the course of the summer therefore a new part is produced upon 

 the outside of the typical vascular bundle which existed in the 

 bud, and the cells of this part are developed at a later period. 



By what we learn both from the increased number of the 

 wood-cells of older internodes as well as from the dissimilar 

 structure of the outer and more considerable portion of their 

 woody bundles, the commonly received opinion, according to 

 which the formation of new wood-cells takes place in the cam- 

 bium layer during the summer, is fully confirmed, and the theory 

 of Harting, which ascribes the extension of the wood in thick- 

 ness solely to the expansion of its cells and the deposition of 

 secondary membrane on the outside of their primary wall, is 

 wholly set aside. 



In reference now to this latter point, the deposition of the 

 secondary layers outside the primary membrane, it would be na- 

 turally very easy to decide the correctness or falsity of this view 



* It is not here meant that lateral connexion between the vascular bun- 

 dles of the medullary sheath is absent in all dicotyledons ; on the contrary, 

 I know well that in many dicotyledons the course of the vascular bundles 

 is quite different from what is stated above, but such connexions are found 

 only at the nodes, and arc altogether wanting in most trees. 



