BARK. 25 



ordinarily of a different colour : it is this part which is commonly 

 called the heart of the wood, and which botanists designate under 

 the name of true-wood, heart-wood, or duramen, while they give 

 the name of albur'mtm or sap-wood to the external ligneous 

 layers, the solidity of which is less, and the colour whiter (Jig- 

 13, c). In other respects the structure of these parts is the same, 

 only the ligneous fibres of the true or perfect wood are filled with 

 solid matters deposited in their interior, while the proportion of 

 liquids is more considerable in the sap-wood or alburnum. In 

 trees of slow growth the line of demarcation is very distinct be- 

 tween I he heart and sap-wood, and in the coloured woods, such 

 as ebony, mahogany, &c., it is the heart only that possesses their 

 peculiar colour, the sap-wood being usually white. In trees of 

 very rapid growth, such as the poplar, willow, &c., there is, on 

 the contrary, but little difference between these two ligneous lay- 

 ers. As we shall see in the sequel, the albur'num is gradually 

 converted into perfect wood, and it is by the formation of new 

 ligneous layers between those already formed and the bark, that 

 the stem increases in thickness. 



38. The medullary rays are the divergent lines which run 

 from the centre of the stem towards its circumference ; they 

 are composed of vertical laminae of compressed cellular tissue, 

 and are very analogous to the pith, from which they seem to 

 arise. These rays come in part from the external ligneous 

 layers, and terminate in the bark, thus establishing a communi- 

 cation between the superficial and central parts of the stem. 



39. The bark is composed first of a layer of cellular tissue, 

 which constitutes the epidermis, and of a deeper layer formed 

 of clostres grouped together so as to form fibres, but without 

 being united with tracheae ; in the progress of age, new alter- 

 nating zones of cellular tissue and fibres, are formed beneath the 

 preceding, and there results from it a series of super-posed lay- 

 ers, which resemble those of the wood, but differ from them es- 

 sentially in their mode of growth ; we have observed that the 

 latter are formed successively one on top of the other ; in the 

 bark, on the contrary, growth takes place from without inwards. 



40. We give the name of liber to the inner layers of the bark, 

 because they are easily detached in thin "plates or laminae, and 

 because the ancients made use of it, as we do paper, to write upon.* 



* Some of our young readers may remember the Latin word, liber, and 

 its several versions, given as follows : 



" Liber, book ; liber, tree ; 

 Liber, child, and liber, free." 



38. What are medullary rays? 



39. How is bark formed ? In what respect does bark differ in its man- 

 ner of growth from wood ? 



40. What part is culled liber 7 

 3 



