THE STRUCTURE OF STEMS. 231 



characters of seed, stem, and leaves. Their seeds are 

 either monocotyledon ous or dicotyledonous. The mo- 

 nocotyledons have stems in which the parts are ar- 

 ranged as we have just shown ; they have the endogen- 

 ous structure, and are hence called endogens. Their 

 leaves are also parallel-veined. The dicotyledons, on 

 the contrary, have stems with the exogenous struct- 

 ure, and are hence called exogens. They have also net- 

 veined leaves. Now, these characters are almost al- 

 ways combined as here stated. There are dicotyledons 

 with parallel-veined leaves, though they are very rare ; 

 but the structure of the stem is characteristic. When 

 you find a plant with a stem having woody bundles 

 surrounding the pith, it belongs to the class of exo- 

 gens ; but, when the woody bundles are seen scattered, 

 without order, through the parenchyma, the plant be- 

 longs to the class of exogens. The coniferae, you 

 remember, are poly cotyledon ous and naked-seeded, 

 but they form their new wood outside the old, and 

 therefore belong to the exogenous class. 



Among the flowerless plants, minute structure 

 also furnishes characters used in classification. The 

 stems of ferns have a mode of growth peculiar to 

 themselves, which has given them the name of acro- 

 gens, or end-growers, because the new parts are always 

 formed above the old. Mosses, algae, and fungi, are 

 called cellular plants, being made up of nothing but 

 cells. It is not till we come to ferns in the ascending 

 scale of vegetation, that any thing like true vessels 

 and fibres appear. 



The scheme on the following page will show you 

 how these plant-characters are used in separating the 

 vegetable kingdom into classes : 



