VEGETABLE KINGDOM DIAGRAM 8. 169 



On looking afresh at the section of the trunk of a tree, we 

 notice concentric rings around its r>ith, which increase in number 

 with the age of the tree. It was formerly supposed that one of 

 these layers was formed under the bark every year, and that their 

 number would indicate the age of the tree ; but it is now known 

 that two or even three layers may be added in one year when the 

 tree is growing rapidly. These layers, which grow annually under 

 the bark of trees, gradually cover up notches which have been cut 

 into the wood, or nails which are driven into it. If, for instance, a 

 nail is driven into a tree which has only twenty concentric layers, 

 as far as the outer part of the pith, it will remain there, because 

 the layers of wood once formed do not alter. But other layers will 

 gradually be added to the first twenty, and afterwards, when the 

 tree is cut down, we shall find the nail in the heart of the wood, 

 without its having stirred from its place. We can also find again 

 in the wood, designs cut into the outer wood, and which have thus 

 been gradually covered by a layer of fresh wood every season. 



The trunks of all trees are not broad near the ground, tapering 

 off to the last branch. Some, like the agave, but more especially 

 the palm-tree, grow to a great height, but are as slender below as 

 above. They only grow in length, and have no concentric rings ; 

 and the wood is not hardest in the centre, but under the bark. 

 Trunks of this description are only found^ among monocoty- 

 ledonous plants. 



The stem or trunk is continued downwards by the root. This 

 has often a conical shape, and strikes into the ground like a pivot ; 

 such are the salsify and the carrot. All trees have roots of this kind. 

 When they are torn up, the central root is seen to separate into a 

 number of branching filaments. These are the organs by which the 

 root pumps up the water from the earth ; and when a tree is 

 transplanted, they ought to be damaged as little as possible. 

 Dicotyledonous plants alone have a root of this kind ; monocoty- 

 ledons never have, and their root is always composed of fibres 

 which are all of equal size, and which all separate from the stalk 

 without ever ramifying. Such is the root of wheat or leek. 



