TEANSVERSE DIFFUSION. 233 



either roots, buds, or leaves, all the material for which must 

 have been derived from the stick itself. (Fig. 22.) 



Similar to this in character is the curious circumstance, not 

 very uufrequent, of old potatoes resolving themselves into sev- 

 eral smaller ones, within the skin of the parent tuber, without 

 any external appearance of vegetation. This is reported to 

 have occurred in a vast number of tubers, in a quantity of po- 

 tatoes on board a vessel in the Arctic ocean, where the low 

 temperature probably exerted some influence in causing this 

 peculiar mode of sprouting. 



An excellent demonstration of the transverse diffusion of 

 sap was obtained in some experiments performed to observe 

 the result of protecting girdled places on trees from the effects 

 of exposure. Healthy young trees, or large branches, of elm, 

 chestnut, apple, grape, and white pine were drawn through 

 glass tubes, two inches in diameter and two feet long, upon 

 either end of which were fastened short pieces of rubber hose. 

 These tubes were placed over girdled spots, from which the 

 bark was removed on the thirtieth of May last, and the rub- 

 ber securely fastened with iron wire to the tree. From all of 

 these specimens a considerable quantity of sap escaped, appar- 

 ently in the form of vapor, and was collected in the tube. 

 There was no layer of wood formed, but the foliage of all ex- 

 cept the pine was killed before autumn, apparently by the 

 fermentation of the sap and its re-absorption into the wood. 

 In the case of an elm root, treated in a similar manner, the 

 bark was renewed, probably from the fact that the cambium 

 Avas in a more advanced state than in the other instances. The 

 root was dug up with care, twenty feet of it drawn through 

 the tube, and then covered again with earth. (Fig. 23.) 



With the view of determining some facts concerning the 

 functions of the bark in connection with the circulation of 

 sap and the growth of wood, many experiments have been un- 

 dertaken at the College during the past two years, and some 

 interesting results obtained. 



In order to learn whether the annual layer of wood upon 

 trees is developed from the outside of the old wood or from 

 the inside of the bark, the following plan, suggested by the 

 interesting experiments of Duhamel more than a century ago, 

 was tried. Vigorous young trees of elm, glaucous willow, 



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