EEPORT OF THE BOTANIST. 173 



Fresh layers had begun to form, not only in connection with the bark 

 both above and below the decorticated portion, but also on the surface 

 of the denuded wood. Messrs. Duhamel, Meyeu, and others have as- 

 serted that when a i)ortion of the wood of the trunk is laid bare- and 

 the decorticated i)ortion is i)reserved from desiccation and from atmos- 

 pheric influences, there are seen exuding from the surface of the wood 

 gelatinous protuberances, mamelons^ as they were named by Duhamel, 

 or gelatinous drops, as they were called by Meyeu. These productions, 

 after they have become multiplied in number, and extended nearer to each 

 other, atr length become hardened and organized as fresh bark. Duha- 

 mel looked upon this as an orgauizable liquid. Meyen noticed that 

 tills exudation, from the first moment when it showed itself at the ex- 

 tremity of the medullary rays, was composed of a very delicate cellular 

 tissue, the cells of which contained a gummy mucilage; but M. Trecul 

 first clearly showed that these new productions are from the beginning- 

 composed of cells, and that these cells, which have a gelatinous appear- 

 ance, are i)roduced by those of the generative layer, which remains on 

 the surface after the bark has been removed. 



Function of the medullary rays. — What is the function of the medullary 

 rays has long been an agitated question, and is one which may receive 

 some elucidation from the facts and observations here stated. It is 

 well known that these rays are but extensions of the cellular tissue of 

 which the pith is composed, and that they are compressed into thin 

 plates by the intrusion of masses of woody fiber. The heart- wood of an 

 old tree becomes filled with earthy deposits, ceases to take part in 

 the circulation, and may perish, as it frequently does in trees 

 with a rotten heart, without impairing the general vitality of 

 the tree, the circulation being carried on through the new outer 

 layers. These medullary rays are extended laterally by cell-multi- 

 plication into every new layer of wood. Is it not x^robable that they 

 retain their vitality only at or near the surface, and that their mul- 

 tiplication at that point creates a net- work of cellular tissue into and 

 through which the descending products of vegetation are diflused "! 

 What is the law which presides over the organization of difiereut 

 tissues is as inexplicable in the vegetable as in the animal system. The 

 medullary rays are identical in nature with the growing point of buds, 

 and their function seems to be the same, viz, the multiplication of cells 

 for the formation of cellular tissue, Avhich is the groundwork of all veg- 

 etable growth. In the case of the denuded wood, exposed to the action 

 of the air and the sun, the medullary rays are dried up and destroyed ; 

 but if they are kept moist, and shielded from the sun, they rapidly en- 

 gage in their special work, and reparation or renewal of the bark is 

 effected, and the formation of other tissues follows as a natural con- 

 sequence. 



The camhium. — " If we examine the stem of a tree in winter or during 

 the period of complete rest, we find between the last-formed layer ct 

 wood and the bark a layer of cellular tissue. At the first return of 

 spring the nourishing sap flows into this generative layer, and swells 

 up its component parts. This zone is composed of tolerably regular 

 cellular tissue. Insensibly, by the progress of vegetation, a k'lrge num- 

 ber of these cells become longer, their cell-walls thicken, and soon pre- 

 sent all the characters of fibrous tissue. Coincideutly with this trans- 

 formation a certain number of cells dispersed in the midst of the others 

 increase in diameter and length, their cell-walls present transparent 

 ininctures, and they become converted into radiated or inmctated ves- 

 sels. These vessels and tubes form bundles separated by a cellular 



