290 HOW CROPS GROW. 



In some of the lower orders of vegetation, as in mush- 

 rooms and lichens, the stem, if any exist, always pre- 

 serves a purely cellular character ; but in all flowering 

 plants the original cellular tissue of the stem, as well as 

 of the root, is shortly penetrated by vascular tissue, 

 consisting of ducts or tubes, which result from the 

 obliteration of the horizontal partitions of cell-tissue, 

 and by wood-cells, which are many times longer than 

 wide, and the walls of which are much thickened by 

 internal deposition. 



These ducts and wood-cells, together with some other 

 forms of cells, are usually found in close connection, and 

 are arranged in bundles, which constitute the fibers of 

 the stem. They are always disposed lengthwise in the 

 stem and branches. They are found to some extent in 

 the softest herbaceous stems, while they constitute a 

 large share of the trunks of most shrubs and trees. 

 From the toughness which they possess, and the manner 

 in which they are woven through the original cellular 

 tissue, they give to the stem its solidity and strength. 



Flowering plants may be divided into two great classes, 

 in consequence of important and obvious differences in 

 the structure of their stems and seeds. These are : 1, 

 Monocotyledons, or Endogens ; and 2, Dicotyledons, or Exo- 

 gens. As regards their stems, these two classes of plants 

 differ in the arrangement of the vascular or woody tissue. 



Endogenous Plants are those whose stems enlarge by 

 the formation of new wood in the interior, and not by 

 the external growth of concentric layers. The embryos 

 in the seeds of endogenous plants consist of a single piece 

 do not readily split into halves or, in botanical lan- 

 guage, have but one cotyledon; hence are called monoco- 

 tyledonous. Indian corn, sugar-cane, sorghum, wheat, 

 oats, rye, barley, the onion, asparagus, and all the 

 grasses, belong to this tribe of plants. 



If a stalk of maize, asparagus, or bamboo be cut 



