ON THE ALBURNUM OF TREES. g<} 



ing trees to prevent their being broken. I then made an 

 incision, more than two lines deep in each, on one side, and 

 at the distance of six or seven lines another incision, equally 

 deep, on the opposite side; and as I am quite certain, from 

 the texture of these branches, that the alburnous tubes 

 passed straight through them, I am equally certain, that every 

 alburnous tube was at least once intersected. Yet the sap The sap still 

 passed into these branches, and their buds unfolded in the buds unfolded, 

 succeeding spring, the incisions having been made in the and the 

 winter. But I have repeated the same experiment after the branches llYed - 

 leaves have been full grown in the summer, and still the 

 branches have contiuued to live. 



All natural ts have agreed in stating, that trees perspire When trees 

 most in the summer, when their leaves have attained their P er fP ire mos,t, 

 full growth, and of course that much sap must ascend a.t tubes dry. 

 this period; yet at this period the tubes of the alburnum 

 appeur dry, and to contain air only ; which induced Grew to 

 suppose, that the sap rose in the state of vapour; a suppo- 

 sition by no means admissible. Yet it is, I conceive, evi- The sap there- 

 dent, that the sap cannot rise as a liquid through dry fore does not 

 » 7 T , rise through 



tubes, nor in any state through intersected tubes ; and there- them. 



fore it appears probable, that it does not rise at ail through 

 the tube3 of the alburnum, and that these tubes are in- 

 tended to execute a different office. 



If the sap do not rise through the tubes of the alburnum, Consequently 

 it must rise through the cellular substance; yet the passage Jt must rise 

 of any fluid through this has been denied by almost every cellular sub- 

 naturalist, probably because coloured infusions have not stance. 

 been observed to penetrate it, and because many naturalists 

 have considered it as mere compressed medulla. Mirbel, 

 however, contends, that the fluid which generates the new 

 bark exudes from it; and although a fluid, capable of pro- 

 ducing the same effects, e:tndes from the bark, when de- 

 tached from the alburnum, 1 am much disposed to coincide 

 with him in opinion, having observed a new bark to be ge- 

 nerated on the surface of the cellular substance of pollard 

 oaks, in detached spaces*. And if the sap in sufficient 

 quantity to generate a new bark can pass through the cel- 



• Phil. Trans. 1607, p. 107 ; or our Journal, vol. XIX, p. 245. 



lular 



