110 



DICOTYLEDONS 



from the air. The hard and dark rings of wood, however, re- 

 present the cessation of growth during the cold and dry season, 

 when the tree drops all its leaves and stretches its Ixire branches 

 towards the brazen skj'. We can, therefore, estimate the age of 

 the tree by counting its rings: for every dark, hard ring corres- 

 ponds to a dry season, and every light, soft ring to a rainy season. 

 Trees of tliis class (Dicotyledons) add cylinders of wood to 

 their trunks every year. This addition of wood is not made in 

 the centre of the trunk, Init under the bark. If the bark is 

 stripped oft" at the time when the tree is growing vigorously, we 

 always tind a sticky watery fluid between bark and wood. This 

 is the sap contained in tender cells which are dividing rapidly. 

 The wood on the outside of the stem is lighter coloured than 

 that in the centre, which is golden yellow when freshly cut. The 

 layers are called sapivood and heartxvood respectively. It is in 

 the outer part of the wood, and not in the old wood in the middle 

 of the tree, that the sap Hows up from the roots to the tips of 

 the branches to produce there, together with the food taken in 



by the leaves, new leaves, flowers and fruits. 

 As new layers are formed one by one every 

 year, the older layers cease to take any 

 active part in the life-work of the tree, 

 and harden. Tlie hardest and most durable 

 wood is the hoartwood which being gradu- 

 ally impregnated by the waste products 

 formed in the course of the growtli of tlie 

 tree, l)ecomes denser and denser. 



2. Tlie Leaves of the Teak tree are 

 very large. They are opposite, and every 

 pair of leaves stands crosswise to the next 

 pair (decussate). In this Avay the load of 

 the great leaves is evenly distributed, as 

 in the Labiatic. We can also see that the 

 stems of young l)ramhes are (quadrangular 

 and channelled, and that they have large ((uadrangular pith. 



The leaves are very rough on their upper side. The lower 

 bide is clothed with dense stellate, gray or tawny liairs. Tliey 



Fif,'. 105. — Portion of a 



iiet-veincil leaf. tn. Stroii; 



midrib, n. Side-rib. 



