THE WONDERS OF VEGETATION. 123 



they bear. South of the Strait of Bab-el-Mandeb 

 (Strait of Tears), near the Gubet-el-Kherab (Basin of 

 Untruth), which is a small bay of that part of the 

 Arabian Gulf called Bahr-el-Bonatien (Sea of the Two 

 Sisters), there stands the little town of Tanjourra. 

 It is here, especially, that the Asdepias gigantea is 

 found growing ; a prickly acacia covered with a num- 

 ber of exuberant lianes. The small antelope, as well 

 as aquatic fowls and the water-hen, haunt the shady 

 woods formed by these beautiful trees ; and this calm 

 and enchanting scene would leave no unpleasant im- 

 pression on your mind, if Tanjourra were not the 

 centre of an abominable slave-trade. 



THE CORK OAK. 



"We will conclude this chapter with a useful plant 

 better known by its peculiar product than in itself. 

 The description of its bark will lead us to consider 

 the general structure of all trees. 



A section of a full- grown tree presents three fun- 

 damental concentric subdivisions. First, the medul- 

 lary canal, containing the pith or medulla. Second, 

 the woody substance surrounding the pith. Third, 

 an outer envelope the bark. In the bark itself there 

 are again three different substances placed in juxta- 

 position ; the liber, consisting of thin leaflets, the 

 parenchyme or cellular system, through which the sap 

 circulates, and the epidermis or outer skin. This is 

 the general structure of all trees. In the tree which 

 produces cork, the parenchyme or middle division of 

 the bark is the portion which furnishes that substance. 



