AND THE STRUCTURE OF PLANTS. 



83 



many of its incurved stems, yet its principal stems measure two thousand 

 feet in circumference, the number of its larger trunks, each exceeding the 

 bulk of our iioblest oaks, amount to three hundred and fifty, while that of 

 its smaller are more than three thousand ; so that seven thousand persons 

 may find ample room to repose under its enormous shade, and may at the 

 same time be richly supplied from the vast abundance of fruit which it yields 

 in its season. 



The solid parts of the trunk of the plant consist of cortex, cuticle, or 

 outer bark ; liber, cutis, or inner bark ; alburnum, or soft wood ; lignum, 

 or hard wood ;* and medulla, or pith. Linneus gave the name of medulla 

 to the pith of plants, upon a supposition that it had a near resemblance to 

 the medulla spinalis of quadrupeds. A closer investigation, however, has 

 since proved that this resemblance is very faint, and that the pith or medulla 

 of vegetables consists of nothing more than a mere spongy cellular sub- 

 stance, forming indeed an admirable reservoir for moisture ; and hence of 

 the utmost importance to young plants, which in consequence of their 

 want of IcE^ves and branches whose surfaces are covered with the bibulous 

 mouths of innumerable lymphatics, would otherwise be frequently in danger 

 of perishing through absolute drought ; but gradually of less use as the 

 plant advances in age, and becomes possessed of these ornamental appen- 

 dages ; and hence, except in a few instances, annually encroached upon, 

 and at length totally obliterated by the surrounding lignum. 



All these lie in concentric circles ; »nd the trunk enlarges, by the forma- 

 tion of a new liber or inner bark every year ; the whole of the liber of one 

 year, excepting indeed its outermost layer, which is transformed into cor- 

 tex, becoming the alburnum of the next, and the alburnum becoming the 

 lignum. Such, at least, is the common theory, and which seems to be 

 well supported by the experiments of Malpighi and Grew : but it has lately 

 been controverted by Mr. Knight, who contends, that the liber has no 

 concern in the formation of new wood, which proceeds from the alburnum 

 alone, a new layer of alburnum being formed for this purpose annually. I 

 cannot discuss the argument at present : nor is it of any great importance ; 

 since, under either system, it is obvious that a mark of any kind which 

 has penetrated through the outer into the inner bark, must in a long pro- 

 cess of years be comparatively transferred to the central parts of the trunk. 

 On which account we often find, in felling trees of great longevity, as an 

 oak, for example, the date of very remote national aeras, and the initials of 

 monarchs who flourished in very early periods of our national history, 

 stamped in the very heart of the timber on its being subdivided. 



Some of these memorials are very curious, and M. Klein, the well- 

 known Secretary of Dantzic, has given various examples in his letter to Sir 

 Hans Sloane, bart., the President of the Royal Society.! One of these 



* There is a curious paper of Count Rumford's mentioned among the labours of the French 

 Imperial Institute for 1812, upon the chemical properties of the different parts entering into 

 the composition of the trunk of trees, for an account of which see also Thomson's Annals of 

 Philos. vol. i. p. 386. By a variety of experiments Count Rumford was led to this singular 

 conclusion, that the specific gravity of the solid matter which constitutes the timber of wood 

 is almost the same in all trees. By the same means he determined that the woody part of oak 

 in full vegetation is only four-tenths of the whole. Air constitutes one-fourth of it, and the 

 rest consists in sap. Light woods have still a much less quantity of solid matter : but the 

 season of the year and the age of the tree occasion considerable variations. Ordinary dry 

 wood contains about one-fourth of its weight of water. Even the oldest wood, though in the 

 state of timber for ages, never Contains less than one-sixth of its weight of water. AH abso- 

 lutely dry woods give from 42 to 43 per cent, of charcoal : whence he concludes, thnt the 

 ligneous matter is identic in all woods. 



t Phil, Traill, for 1739, vol. xli. p. m. 



