FUNCTIONS OF PLANTS. 31 



the vessels in the under surface of the leaves, whence it descends to the roots, 

 not however by way of the alburnum, where it would meet with and inter- 

 rupt the ascending sap ; but by way of the inner bark, communicating hori- 

 zontally, as we have before observed, with the interior of the stem by means 

 of the medullary rays. Hence, the great importance of the alburnum and 

 the inner bark to plants ; the former in conveying sap from the root to the 

 leaves, and the latter in returning it from the leaves to the stem, branches, 

 and roots. Hence also we find that trees will live, and even thrive, with the 

 interior of their trunk entirely rotten, provided the alburnum, the inner 

 bark, and the leaves, are in a healthy state. The alburnum is constantly 

 changing into hard wood, and the inner bark as constantly into hard bark or 

 outer bark. As the heart wood when thoroughly hardened may be removed 

 without injury to the growth of the tree, so also may the thoroughly hard- 

 ened outer bark. The hard wood is to the tree what the bones are to an 

 animal, the chief source of mechanical support ; and the outer bark being a 

 non-conductor of heat, protects the inner bark and the alburnum from too 

 great cold, and in hot climates from too much heat, in the same manner as 

 the outer coverings of animals. 



118. Though the sap of plants circulates in general by rising through the 

 alburnum, and descending through the inner bark, yet such is the effect of 

 vitality, and the simplicity of their structure, that the sap can be made both 

 to rise and fall by the alburnum, and to rise and fall also by the inner bark. 

 Instead of ascending from the roots to the branches, it can be made to enter 

 by the branches and descend to the roots. To prove the truth of the first of 

 these assertions, the trunk of a tree has been sawn through in opposite direc- 

 tions in such a manner that there could not, by any possibility, be direct 

 linear communication between the portions below and above the wound, and 

 yet the tree has lived. The wood of the shoot of a willow has been extracted 

 at the peeling season, and the shoot being supported by a stake, has grown, 

 and in the course of the first summer filled up the cavity left by the removal 

 of the wood. That the sap will both ascend and return, not merely by the 

 alburnum, but by wood of a considerable degree of age and hardness, is 

 proved, among other instances, by a Lime-tree in the royal gardens at Fon- 



tainebleau, which continues to live and produce leaves every 

 year, though a large portion of the stem has been without 

 bark for thirty years. Fig. 1 is from a sketch made by 

 M. Poitcau, a scientific cultivator and physiologist, in 

 whose company we examined this tree in July, 1840. To 

 prove that the sap will enter by the branches and descend 

 to the leaves, take a ligneous plant growing in a pot, and 

 elevating it on a post between two trees of the same or 

 of allied kinds, inarch the extremity of a branch of each 

 tree into the plant in the pot, and in two years cease to 



, supply water to the earth in the pot, and at last shake this 

 Part of the trunk of *A J c , , * , 



a Lime Tree grow- eartn awa ^ from the 1>oots > mid leave tne P lant S us P e nded 



ing at Fontaine- between the two trees. We have not seen this done, but 



blcau in 1840. we have seen branches which had inosculated with other 



branches cut through, and being left attached by the inosculation, live for 



several years. Some curious experiments bearing on this subject, by Mr. 



Niven of Dublin, will be found in the Gardeners Magazine, 1838, p. 161. 



119. The cause of the motion of the sap is a subject which has occasioned 

 much discussion. The general opinion is, that it is in motion, to a certain 



