ON THE ORIGIN OF ALBURNUM. 149 



Many different hypotheses have been offered by naturalists to account 

 for the force with which the sap ascends in the spring ; of these hypo- 

 theses two only appear in any degree adequate to the effects produced. 

 Saussure, jun. supposes that the tubes contract as soon as they have 

 received the sap in the root, and that this contraction, commencing 

 in the root, proceeds upwards, impelling the sap before it : and I have 

 suggested that the expansion and contraction of the compressed cellular, 

 or laminated substance (the tissu cellulaire of Duhamel and Mirbel) 

 which expands and contracts with change of temperature* after the tree 

 has ceased to live, might produce similar effects by occasioning nearly a 

 similar motion and compression of the tubes, the coats of which are, 

 I believe, universally admitted not to be membranous. But both these 

 hypotheses are inconsistent with the facts that I have now the pleasure to 

 communicate to you. 



Selecting parts of the stems of young trees from which annual branches 

 had sprung in the preceding year, I ascertained, by injecting coloured 

 infusions into the stems through the annual shoots, that the tubes which 

 descended from the latter, were, at their bases, confined to that side of 

 the stem from which they sprang, and to the external annual layer of 

 wood. Deep incisions were then made into the stems of other trees 

 immediately beneath the bases of similar annual shoots, by which I am 

 quite confident that all communication through the alburnous tubes, with 

 the stem, was wholly cut off; yet the sap passed into the annual shoots 

 in the succeeding spring, all of which lived, and some grew with consider- 

 able vigour. I, at the same time, selected many lateral branches, 1 ' about 

 three lines in diameter, in a nursery of apple trees, which I could ^easily 

 secure to the stems of the adjoining 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 passed into these branches, and their buds unfolded in 

 the succeeding spring, the incisions having been made in the winter. 

 But I have repeated the same experiment after the leaves have been 

 full-grown in the summer, and still the branches have continued* to 

 live. 



All naturalists have agreed in stating that trees perspire most in 

 the summer, when their leaves have attained their full growth, and of 



* See above, p. 92. 



