EECKEATIVE SCIEN'CE. 



63 



or alburnum, with tlie bark, is in truth, tlie 

 growing part of the tree, which, being con- 

 stantly deposited from the bark and inner 

 bark, becomes in its turn conyerted into 

 wood or timber. 



Spring giyes a new impulse to vege- 

 table life, and the living juices of trees, no 

 less than the current in our own veins, feel 

 the influence of the genial ray. The absence 

 of that amount of moisture in the upper part 

 of trees which occurs in winter, causes the 

 cellular tissues to become, as it were, vacua, 

 and the sap rises by what is called capil- 

 lary attraction. 



. Of the specific gravity of woody fibre, 

 water forms usually two-thirds ; but woods 

 difier extremely in this particular as well 

 as in their contractile power, which de- 

 pends entirely upon the direction of the 

 fibres. The lighter the wood, the greater ca- 

 pacity it has for imbibing moisture, and some 

 trees secrete a vast amount, either of watery 

 sap, as in the JBetula alba, or white birch, or 

 gum, as in the caoutchouc (Siphonia elas- 

 tica) ; others, such as Brazil-wood, log-wood, 

 etc., have very useful qualities for staining 

 by the juices that they secrete. Others, such 

 as mahogany (^S^y^e^e«^« ?wa7^o^(^M^), rose-wood, 

 which is produced by a species of Mimosa, 

 king-wood, etc., are coloured by these juices, 

 and are used in ornamental work. Satin- 

 wood [Chloroxylon Stoietenia) is well known, 

 and yields a wood-oil, as also does the sandal- 

 wood (Sandonicum) of the family 3£eliaceai ; 

 ebony {Ebenacece) and calamander-wood (of 

 Ceylon) arc examples of this kind. With 

 the characters of the fir tribe we are weU 

 acquainted, it being a matter of notoriety 

 that they yield all the resins, and that tar is 

 made from their roots. 



With regard to the rising and falling of 

 the sap, a familiar illustration has often been 

 given in proof of the alleged fact, that there 

 is a much greater quantity of sap in the upper 

 than in the lower part of a tree ; namely, 

 that if a piece of the bark is cut out trans- 



versely in spring, the sap exudes in much 

 greater quantity from the upper part than 

 from the lower. This is owing to the fact 

 that the sap current is upward in the wood 

 and downward in the bark. 



The last portion of the tree to be consi- 

 dered is the bark. This substance, which 

 varies from a mere thin skin to a coating a 

 foot in thickness, as in the case of the Dou- 

 glas Fir of America, is composed of several 

 parts. The innermost (we have been proceed- 

 ing from the centre) is called the liber, and a 

 soft viscid substance, called cambium, or pa- 

 renchyma, which form what are called the cor- 

 tical layers, or layers of the bark. Through 

 this substance the ligneous cords and medul- 

 lary processes pass, and it is perforated by 

 cellular tissue ; wood being, in fact, a col- 

 lection of tubular and vascular tissue. Outer- 

 most of all is the true bark, the epidermis, or 

 cuticle, through which the atmosphere acts 

 on the plant ; not referring to the leaves, 

 which forms matter for separate considera- 

 tion. The bark is, in fact, an organization 

 to inspire and expire by, but whether the 

 wood is formed by or from the leaves is 

 a vexed question. The leaves of exogens 

 ramify from a centre or midrib, and are 

 netted in the most exquisite manner ; whereas 

 those of endogens, as I have before observed, 

 grow in parallel lines. Endogens are so 

 called because there is a continual develop- 

 ment towards the interior, and we find them 

 spoken of by Theophrastus. Several elabo- 

 rate works have been published on this class, 

 to which the grasses belong. Endogens some- 

 times terminate in a single shoot, whereas 

 exogens are furnished with buds which are 

 axillary, or resembling arteries from the main 

 vein. When the spring-growth in trees 

 takes place, there is a spontaneous separation 

 between the liber and the bark, which ia 

 necessary for the growth of the wood ; an 

 addition to the wood is taken from the liber, 

 and the cortical layers again receive an 

 external addition, and so the tree progresses. 



