168 VEGETABLE KINGDOM DIAGRAM 8. 



cotyledons, swelling in the damp earth, finally split the envelope ; 

 the small plant then frees itself, though still remaining attached to 

 the cotyledons ; the bud rises into the air, and becomes the stem 

 and leaves, and the root strikes into the earth. The two coty- 

 ledons still remain attached to the plant between the stem and the 

 root for a short time, like two large leaves, by a thick stalk, but 

 afterwards fall off. 



Plants have never more than two cotyledons, but a great 

 number have only one. Plants which have two are called 

 Dicotyledons, and those which have only one, Monocotyledons ; 

 while plants which have none at all are designated Acotyledons. 

 Acotyledonous plants differ very much from others; they are 

 mosses, fungi, lichens, &c. Dicotyledonous and monocotyledonous 

 plants also differ from one another, so that in most cases there is 

 no occasion to count the cotyledons of the seeds to know in which 

 class they should be placed, for the trunk, leaf, and flower show 

 plainly if the plant belongs to one or other of the two divisions, as 

 we shall see by the examples which we shall give further on. 



The Trunk. When the stem of the plant is of large dimensions, 

 it is called the trunk. Trees have a trunk which is sometimes so 

 large that a man can get into it when it is hollow. 



If we saw through the trunk of a tree about a yard from the 

 ground, we first notice that it is formed of three parts ; the bark, 

 the wood, and, lastly, the pith in the centre. The pith is very 

 light and soft, and is surrounded by the medullary sheath, which is 

 often very small in timber trees, but it is sometimes much larger, 

 as for instance, in the branches of the elder, where the pith fills 

 more space than the wood. 



The Wood itself forms two layers, which can easily be distin- 

 guished ; the inner layer fs darker than the other, and much 

 harder. This is called the heart of the wood. The outer portion 

 is less dark and hard. The difference between the heart of the 

 wood and the outer layer, or sap-ivood, is more conspicuous in some 

 woods than others. Ebony, for instance, is the heart of a tree, the 

 outer layer of which is white, like deal. 



