447 



• 



nected. The nniou of roots in these cases was sbmetimes woody and 

 sometimes only by tlie bark of the roots. So far as observed this 

 anastomosis or natural grafting- is confined to coniferous trees, and to 

 only a few species of them, chiefly the silver fir, the spruce, and occa- 

 sionally the Scotch fir. In the London Gardeners' Chronicle of August 

 31 is an account of an instance of this kind of anastomosis of the roots 

 of a larch, and a figure is given of the specimen, in which the stump 

 and its root connections are exhibited. The cut stump shows rotten 

 wood in the center, with the new wood at the circumference surging 

 over the edges of the wound. 



Although the discoA'cry of this root union explains some of the ques- 

 tions involved in this curious pheuomenon, it does not explain them all f 

 for instance, why does not the sap, which is thus robbed from the roots 

 of the nurse tree, pass up in the usual channels and overflow at -the top 

 of the stump, as is the case when a grape-vine or deciduous tree is cut 

 during the active ascent of the sap '? As the growth of new wood in 

 exogenous trees takes place from the cambium, and the cambium is 

 supposed to be the sap which has been elaborated in the leaves, what 

 is the source of the cambium in these stumps ? 



It would seem as if there was here a complete contradiction of the 

 ingenious theorj^ of some of the French botanists, that wood growth 

 begins in the leaves or leaf-buds and descends continuously from thence 

 to the roots, so that in fact wood may be considered the united mass of 

 roots which emanate from the leaves of the plant. 



The theory of De Candolle is that the woody and cortical layers orig- 

 inate laterally in the cambium furnished by pre-existing layers and 

 nourished by the descending sap. To use the words of De Candolle, 

 " The whole question may be reduced to this : either there descend from 

 the top of a tree the rudiments of fibers which are nourished and devel- 

 oped by the juices springing laterally from the body of the wood and 

 bark, or new layers are developed by i)re-existing layers which are 

 nourished by the descending juices formed in the leaves." The latter 

 j)art of this statement, though somewhat vague and unsatisfactory, 

 probably involves the true theory of the formation of wood. The pre- 

 existing layers mentioned in De Candolle's statement include the medul- 

 lary rays which reach the circumference. These medullary rays are 

 composed of cellular tissue derived from the pith, and, like it, are capable 

 of indefinite extension by cell multiplication. 



The primary state of all the tissues of the jilant is the condition of 

 simple cells, each of which is in a certain sense an independent body, 

 having its own life-work and history in the complicated mass of which 

 the tree is composed. All exttmsion of the tree in any direction is made 

 through the medium of cell-growth and cell-modifications, and wher- 

 ever there is cellular tissue in a state of vitality there may be cell-mul- 

 tiplication whenever material for growth, i. e., sap in different stages, is 

 brought into contact with such tissue. 



In the case of the pine stumps alluded to, the medullary rays of the 

 recent wood retain their vitality, and when the sap rises it is transmit- 

 ted through these rays and through the interspaces of the woody matter 

 to the surface beneath the bark, these being appropriated to the organ- 

 ization of new cells whose walls are thickened by continuous secondary 

 deposits, as in the normal formation of woody tissue. Of course the 

 amount of this woody formation will be limited, from the deficient sup- 

 ply of sap and the want of concentration which it would obtain by pass- 

 ing through the leaves. 



