THE SECOND BOOK OF BOTANY. 



and form a zone of a sort of netted liber inside the 

 cellular layer, which serves as bark. 



It is thus apparent that the central part of the 

 stem will always be made up of the part of the 

 woody thread in which the wood predominates. This 

 part of the thread is porous, and contains great ves- 

 sels, so that the centre of the stem is rather cellu- 

 lar and vascular than fibrous. In the same way the 

 peripheral part of the stem always contains, or is 

 made up chiefly of, the part of the thread in which 

 the cortical portion, the bast-cells, or liber, predomi- 

 nate ; it is, therefore, more solid. Outside of this is 

 the region where the spongy bundles split and inter- 

 lace, losing themselves in the bark ; so that an hori- 

 zontal section of a monocotyledon ous stem is made 

 up of a central porous portion, a peripheral colored 

 and dense portion, and a zone of a sort of liber ex- 

 terior to this. In dicotyledons, on the contrary, the 

 old, solid wood is in the centre, or heart, of the stem, 

 and the new, soft wood surrounds it. A monocoty- 

 ledon ous stem presents nearly the same size along 

 its whole length. This is because the woody bundles 

 lessen gradually toward their inferior portion, and are 

 not all collected at the base of the tree, as in dicoty- 

 ledons. 



In monocotyledons, the new wood is formed in 

 the central part of the stem ; they are hence called 

 endogens, or inside growers, while dicotyledons, which 

 form their new wood in circles outside the old, are 

 called exogens, or outside growers. 



This is, perhaps, as good a place as any to tell you 

 that all plants that bear their seeds in closed seed-ves- 

 sels may be divided into two great classes, based upon 



