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RISE AND FALL OF SAP. 



POPULAR ERRORS ABOUT THE "RISE AND FALL " OF SAP. 



BY PROP. LINDLEY, UNIVERSITY OF LONDON. 



What a curious hallucination is that which 

 supposes the sap of trees to fall, or set- 

 tle, in the winter into the roots ! One 

 would have thought that the notorious diffi- 

 culty of cramming a quart of water into a 

 pint measure, might have suggested the im- 

 probability of such a phenomenon. For it 

 certainly does require a very large amount 

 of credulity to believe that the fluids of the 

 trunk and head of a tree, can by any natu- 

 ral force of compression, be compelled to 

 enter so narrow a lodging as the root. The 

 idea, however, has established itself in. some 

 persons' minds, and, we presume, in con- 

 nection with that other old vulgar error, that 

 the sap is in rapid motion in the spring 

 time, in the roots of a tree, before it begins 

 to flow in the branches. These whimsies 

 took their origin in days when the world was 

 contented to accept assertions upon trust, 

 and when hypotheses and vain imaginings 

 formed the debased paper currency of sci- 

 ence. But now men have found out the 

 value of a golden standard, both for money 

 and for knowledge ; they call for facts be- 

 fore theories ; and the result, already, is a 

 wonderful disturbance in the crowded ranks 

 of scientific as well as historical legends. 



We shall assume the word sap to signify 

 the fluids, of whatever nature, which are 

 contained in the interior of a tree. In the 

 spring, the sap runs out of the trunk when 

 it is wounded ; in the summer, autumn, and 

 winter, it does not, unless exceptionally, 

 make its appearance. But in truth the sap 

 is always in motion, at all seasons, and un- 

 der all circumstances except in the presence 

 of intense cold. The difference is, that 

 there is a great deal of it in the spring, and 

 much less at other seasons. 



When a tree falls to rest at the ap- 

 proach of winter, its leaves have carried ofi 

 so much more fluid than the roots have been 

 able to supply, that the whole of the inte- 

 rior is in a state of comparative dryness; 

 and a large portion of that sap which once 

 was fluid, has become solid, in consequence 

 of the various chemical changes it has un- 

 dergone. Between simple evaporation on 

 the one hand, and chemical solidification on 

 the other, the sap is, in the autumn, so 

 much diminished in quantity as to be no 

 longer discoverable by mere incisions. The 

 power that a plant may possess of resisting 

 cold, is in proportion to the completeness of 

 this drying process. 



When the leaves have fallen off, the tree 

 is no longer subject to much loss of fluid by 

 perspiration, nor to extensive chemical 

 changes by assimilation, for the leaves are 

 the principal organs of perspiration and as- 

 similation. But the absorbing power of the 

 roots is not arrested ; they, on the contrary, 

 go on sucking fluid from the soil, and driv- 

 ing it upwards into the system. The effect 

 of this is, that after some months of such 

 an action, that loss of fluid which the tree 

 had sustained in autumn by its leaves, is 

 made good, and the whole plant is distend- 

 ed with watery particles. This is a most 

 wise provision, in order to insure abundant 

 food to the new-born leaves and branches, 

 when warmth and light stimulate them into 

 growth. 



During all the winter period, the sap ap- 

 pears indeed to be at rest, for the re-filling 

 process is a very gradual one. But M. Biot, 

 many years ago, proved, by an ingenious 

 apparatus, that the rate of motion of the 

 sap may be measured at all seasons; and 



