62 



EECREATIVE SCIENCE. 



endogens, in a single bud. Various opi- 

 nions have been formed as to the mode in 

 wbich these fibres were originally formed, 

 some botanists contending that each filament, 



or each, bundle of fibres, is, in fact, a root of 

 a leaf, and this is strengthened by the fact 

 that in the endogens the fibres forming the 

 leaf lie side by side in parallel lines. The 

 leaves of this family of plants are sometimes 

 of great size, extending to as much as forty 

 feet in width — enough to cover a dinner- 

 table with an extempore green cloth I En- 

 dogens contain a great proportion of trees 

 yielding fruit applicable for food, and, per- 

 haps, fewer poisonous plants than the exogens, 

 and there is a remarkable contrast in size in 

 some specimens. When we come to consider 

 the exogens, we find amongst them all those 

 trees which we may truly call " timber," and 

 which are found in such numbers and varie- 

 ties in Old England, not to mention others 

 which have been found capable of existing 

 in an English climate. In this class we have 

 what may be truly called " wood," and de- 

 fined as that portion of the tree existing be- 

 tween the pith and the bark. ' Botanists 

 regard this substance as bundles of woody 

 tissue or fibre, but, taking a timber tree as 

 a whole, it is divided as follows : — The cen- 

 tre, pith ; next to that, heart-wood ; then sap- 

 wood ; then bark. The pith is a cellular sub- 

 stance, large in comparison to the size of the 

 plantwhen very young ; this comparison, how- 

 ever, is soon lost as the young plant becomes 

 a tree, and in old age is nearly indiscernable. 

 Linnajus — that father of naturalists, whose 

 works, imperfect as they are, having relation 

 to modern discovery, will long stand unri- 

 valled — amongst other theories, considered 

 that the pith held the same place in trees 

 that marrow does in the himian frame, and 

 was, in fact, the centre and soul of the 



life ; this, however, was never, I believe, fol- 

 lowed, because it was untenable ; and, there- 

 fore, at the present day, pith may be defined 

 as a cellular substance found in the centre of 

 the bole and branches of a tree, but not of 

 the roots (that is, of exogens). When 

 young it is filled with fluid and grains of 

 starch, which disappear when the foliage be- 

 comes organized, and so serves as a magazine 

 for nutriment. The supposition is, that at this 

 period its office is performed, that it then 

 dies, and, gradually contracting in bulk, at 

 length entirely vanishes. There is no doubt, 

 however, that a communication is kept up 

 between the centre and circumference of tim- 

 ber-trees, as the rays which radiate to the 

 bark seem to prove. These are called me- 

 dullary, or marrow-like (founded, perhaps, on 

 the fact that they communicate with the cen- 

 tre), and these again communicate with the 

 leaves by cords of a fibro-vascular tissue. 

 The heart- wood, which is resident next to the 

 pith, forms, towards the centre, the true 

 timber, and the support of the tree by its 

 firmness and solidity. This increases in 

 density and hardness as years pass away, 

 partly, no doubt, by age, and partly by accu- 

 mulated pressure from without. Heart- wood 

 may be regarded as comparatively a dead 

 substance, although it cannot be denied that 

 it always contains some moisture or juices, 

 but these arc not in active circulation. 

 Whilst the tree is a growing tree, the mode 

 in which it is formed is by a constant anniial 

 deposit and outward growth, by which layer 

 upon layer being added, the tree gradually 

 increases in bulk, and those rings are formed 

 which we see at the end of a fresh-cut stick 

 of timber. External to the heart-wood is the 

 sap-wood, or alburnum, which may be re- 

 regarded as the heart-wood in a young or 

 progressive state. The circles seen in the 

 heart-wood are known as spurious grain, and 

 are in fact nothing more than the junction of 

 the annual deposits. The medullary rays are 

 also known as silver (/raifi. The sap-wood, 



