LEAYES. * 129 



From the pith in the centre, through the woody part, rays 

 or laminae of cellular tissue similar to that which constitutes 

 the pith itself, are sent outwards through the woody rings 

 to the inner part of the bark, which they form ; these are 

 called medullary rays, and may be % seen in wood which has 

 been cut across the grain, they are often called the " silver 

 grain," and are very evident in oak, beech, and elm, the inner 

 part of the bark is called the " liber." The bark itself is made 

 of cellular tissue dried and hardened by age ; the outer 

 portion (called the "epidermis") is in many cases shed, and 

 may be constantly seen hanging loosely from the bark of the 

 birch and other trees in loose white silvery portions. The 

 outer part of the bark of some trees is so largely developed 

 as to be of considerable thickness, this is especially the 

 case in the Cork oak (Qwercus suber), which is the cork of 

 commerce. The bark cannot stretch as the circumference 

 of the tree increases, it is therefore split up and cracked, 

 which accounts for the rough state of it on those trees 

 which do not shed the outer part. 



The chief appendage of the branches, is called a leaf, it 

 grows from a small projection called a leaf-bud, which 

 contains the leaf rolled up. The method in which this 

 occurs is different in different plants, in some it is folded 

 backwards and forwards, in others doubled up with the 

 opposite leaf alternately,and in various forms in other plants. 

 The leaf consists of two parts, the stalk (petiole) and the 

 blade (lamina) ; the blade is of different forms, and has ribs 

 and veins covering it, in some of a reticulated or network 

 pattern, these belong to the exogenae, and in others running 

 parallel, which is the kind of venation found in the leaves of 

 the endogenae. When leaves are placed on a stalk, they are 

 said to be "petiolate," when without one, " sessile." 



When leaves are not separated into different portions 

 (although they may be much notched) they are said to be 

 " simple," as in the oak or willow, but when divided into 

 separate portions, as in the ash, they are said to be 

 "compound." 



The following are the chief forms of leaves, named 

 according to variations of their several parts : 



