OK, PLAIN 



extensible with the growth of the 

 trees, until it reaches its natural 

 dimensions. The pith may be 

 said to prevail throughout the 

 stem, and to form the bed into 

 which the woody fibres pass. 



9 



562. 



As the leaves are the true 

 wood-forming organs, it may be 

 concluded that the amount of 

 timber in a tree has a relation 

 to the number of leaves it has 

 borne. The surface of leaves is 

 commonly marked by a number 

 of ridges, which are called veins. 

 These consist of woody tissue, 

 spiral vessels, and cellular tissue ; 

 and the intervening spaces be- 

 tween these veins are filled with 

 cellular tissue. The veins are 

 brought in closer proximity in 

 the midrib and stalk of the leaf 

 1 ; and having passed through it 

 into the stem, one part enters the 

 bark, whilst the other traverses 

 the wood, and penetrates to the 

 medulla?^ sheath, which encloses 

 the pith. Every leaf is therefore 

 9* 



TEACHING. 20 1 



a prolongation of the pith, spiral 

 vessels, and wood of the stem. 



The distribution of veins in 

 leaves is according to a uniform 

 plan : the venous leaf-systems of 

 exogenous plants differing mate- 

 rially from those of the endoge- 

 nous. The veins of an exogenous 

 leaf, 10, consist of a central mid" 

 rib, I, a series of festoons ar- 

 ranged on either side of it, con- 

 stituting the primary veins, 2 ; 

 the curves by which they com- 

 municate with each other are the 

 curved veins, 3 ; from the curved 

 veins other and smaller vessels 

 are given off, called the external 

 veins, 4, and from these smaller 

 vessels are distributed to the 

 margin of the leaf, forming the 

 marginal veinlets, o. Within the 



10 



563. 



primary festoons are small ves- 

 sels proceeding from the midrib, 

 called the cortal veins, 6 ; the 

 small branches from the primaries 

 are called proper veinlets, 7, while 

 the branches of those which com- 

 municate with each other are the 

 common veinlets, 8. 



It must not be supposed that 

 these systems of veins can be 

 traced in all leaves, for in the 



