ASCENT OF THE SAP IN TREES. 91 



after it has dropped its leaves in the autumn, it will instantly vegetate. 

 This effect appears to me to arise from the latter plant's possessing a 

 degree of irritability, which has been exhausted in the former, by the 

 heat of the stove, but which it will acquire again during the winter, or by 

 being drawn out, and exposed for a short time to the autumnal frost. 

 On the same principle, we may point out the cause why seedling plants 

 always thrive better in the spring than in the autumn, though the weather 

 be apparently less favourable. In the former season, the stimulus of heat 

 and light is gradually becoming greater than that to which the plant has 

 been accustomed ; in the latter season, it becomes gradually less. 



There is another circumstance attending trees that have been made to 

 blossom early in the preceding spring, which has always appeared to me 

 an extremely interesting one. If a peach-tree, for example, be brought 

 into blossom in one season in the beginning of February, by artificial heat, 

 it will spontaneously show strong marks of vegetation at the approach of 

 that season in the succeeding year ; and, if it be not well protected, it 

 will expose its. blossoms to almost inevitable destruction. I do not see 

 any cause to which this effect can be attributed, except to the accumu- 

 lated irritability of the plant. 



That heat is the remote cause of the ascent of the sap cannot, I think, 

 be doubted ; and, perhaps, frequent variations of it are, in some degree, 

 requisite ; (for plants have always appeared to me to thrive best with 

 moderate variations of temperature ;) but the immediate cause will, I 

 think, be found in an intrinsic power of producing motion, inherent in 

 vegetable life ; and I hope to be able to point out an agent, by which the 

 mechanical force required may possibly be given. 



There is, you know, in every kind of wood, what workmen call its grain, 

 consisting of two kinds, the false or bastard, and the true or silver grain. 

 The former consists of those concentric circles which mark the annual 

 increase of the tree ; and the latter is composed of thin laminae, diverging 

 in every direction from the medulla to the bark, having little adhesion to 

 each other at any time, and less during the spring and summer than in 

 the autumn and winter ; whence the greater brittleness of the wood in 

 the former seasons. These laminae (which are of different width in dif- 

 ferent kinds of wood) lie between, and press on, the sap vessels of the 

 alburnum ; they are visible in every wood that I have had an opportunity 

 to examine, except some of the palm tribe ; and these appear to me to 

 have peculiar organs, to answer a similar purpose. If you will examine a 

 piece of oak, you will find the laminae I describe ; and that every tube is 

 touched by them at short distances, and slightly diverted from its course. 



