MEDULLARY RAYS THE BARK. 



77 



b 6 



Fig. 131. Vertical 



(Fig. 128). This sheath has no special walls, but is simply bounded by the pith on the 

 inner, and by the wood (when it exists) on the outer side. It is in this situation that 

 we may find ducts of various kinds, and spiral vessels ; and in all cases it conveys the 

 vascular structure from the root direct to each leaf and flower. The integrity of this 

 structure is therefore highly necessary to the life of the plant. It is said to retain its 

 green colour to the latest period of the existence of the plant; thus showing the impor- 

 tance of the functions assigned to it. 



Medullary Rays. These structures come next in order ; and, as has been already 

 intimated, belong to the horizontal cellular system of the stem. They constitute the 

 channels of communication between the bark and pith, and are composed of a series of 

 walls, of single muriform cells resting upon the root, and pro- 

 ceeding to the apex of the tree, and radiating from the centre. 

 They lie between the wedge-like blocks of wood, and, as 

 they have a lighter colour than the wood, they are evident 

 on a section of any stem, and are called the silver grain 

 (Fig. 131). Their colour and number suffice to enable us to 

 distinguish various kinds of wood, and greatly increases their 

 beauty. They cannot, of course, exist before the wood is 



of an egenou formed, and are therefore not met with in the earliest condi- 



across the medullary tion of the plant. They begin to exist with the first deposited 

 ravs, showing their open /. -, ^ -n 



character and their rela- Ia 7 er of wood, and continue to grow outwardly, or nearest to 

 tive position to the wood, the bark, so long as the wood continues to be deposited. 



a. Dotted duct. , i -, i -n -, i . 



b. Woody fibre. The Bark. As the medullary rays terminate in the bark 

 e. End of medullary rays. on their outer s {^ we are naturally next led to a considera- 

 tion of that structure. It forms the outer covering a sheath of the tree, and, in some 

 form or other is present in all plants. "When discussing the constitution of the cuticle 

 of herbaceous plants we explained the points of difference between the two varieties 

 of the same structure, and showed that the rudeness of the bark of wooded trees had 

 destroyed many of the characters of the cutis, such as stomata and hairs. We have 

 now to regard it as a dense cellular organ, well fitted to endure the influences of sea- 

 sons through a long series of years. 



It may anatomically be divided into two structures viz., an outer one, which is 

 cellular, and an inner one, which is vascular or woody. The former is sub-divisible 

 into three parts, whilst the latter is composed of several layers of the same material, 

 and forms a link between the wood and the bark. 



The three divisions of the cellular part are the Epidermis, the Epiphlaoum, and the 

 Mesophlaeum. 



The Epidermis is the most external layer, and is continuous with that upon the 

 leaves. Its cells are flattened and lengthened, and but very rarely possess stomata. 



The Epiphleeum has acquired much importance from the fact of its being the part of 

 the bark in which the cork is deposited. It cracks and peels off at intervals in almost 

 all trees. In the birch and cherry it may at all times be seen hanging from the stem 

 in silvery shreds, and in other trees as rough broken patches. In the cork tree (Qnercus 

 suber] it remains firm until the tree has attained a certain age, after which it exfoliates 

 in the large masses in which it is brought to this country. It is probable that the 

 deposition of cork proceeds in all trees ; but in the cork tree it attains so great a thick* 

 ness as to become a highly important article of commerce. 



The removal of the cork from the cork tree is not left to natural exfoliation ; but, when 



