ORGANIZATION OF THE STEM. 47 



leaves with which they are terminated superiorly, and from 

 them the new vegetable matter descends along the centre of 

 the stem, and pushes outward the parts first formed. 



The oldest and hardest part of the stem of an endogen is 

 that nearest the circumference ; for the more the external parts 

 are pressed by the descent internally of new vegetable matter, 

 the denser must they necessarily become. It is owing to 

 this external hardness that many endogenous plants have no 

 lateral buds or branches, because they are unable to penetrate 

 the hard casing of the stem. 



On the cross-section, the stem of an endogen is not distin- 

 guishable into bark, wood, and pith, neither does it present 

 any appearance of concentric rings; for the stem of an endo- 

 gen is composed of separate bundles of vascular or woody 

 tissue irregularly imbedded in a mass of cellular tissue, which 

 bundles are distinctly traceable down into the stem from the 

 base of the leaves at its summit, and then curving outwards 

 they generally terminate in the bark. Hence on the cross- 

 section the cut ends of these bundles are visible in the form 

 of dots, interspersed through the uniform cellular tissue, with- 

 out any apparent order, although more commonly crowded 

 towards the circumference. Hence, also, we see the reason 

 why the bark of an endogen is inseparable from the rest of the 

 stem without a laceration of its fibres. 



The plants whose structure is endogenous in the northern 

 United States are few, and, with the exception of the green 

 brier, entirely herbaceous. The grasses, the iris, the Indian 

 corn, are humble representatives of endogenous plants, which 

 attain their full development and display their noble arborescent 

 forms only under the influence of a tropical sun. The palms, 

 screw pines, plantains, and bananas of the tropics, are all endo- 



