GO 



CHAPTER XI. 



THE STEM, OR ASCENDING AXIS. 



IGl. That part of the plant which originates with the phim- 

 xile (124, b), and arises above the surface, expanding itself to 

 the iniluence of the air and the light, is called the ascending 



AXIS or STEM. 



a. Tlie cause of its upward tendency is unknown |13], note), but is supposed 

 to be in some way due to tlie principles of light and gravitatiou. ) 



162. Although the first direction of the stem's growth is verti- 

 cal, there are many plants in which it doe^ not continue so, but 

 extends in an oblique or horizontal direction, either just above 

 the surface of the ground, or just beneath it. }' When the stem 

 continues to arise in its original direction, it is said to be erect. 

 When it groAvs horizontally upon the surface, it is said to be 

 procumbent, creeping, trailing, &c. , When it arises obliquely it 

 is an ascending stem, and when it continues buried beneath the 

 soil it is a subterranean stem. 



a. The subten-ancan stem, and some varieties of the creeping, have usually 

 been described as roots. 



1G.3. In regard to duration, the stem, like the root,, is said to 

 be annual wdien it lives but one season, afterAvards dying, at 

 least down to the root, and peren?iial when its existence is con- 

 tinued beyond one season, to an indefinite period of time.- 



1G4. In regard to the si/e and duration of the stem, plants are 

 distinguished into trees, shnibs, and herbs. 



a. A TREE is a plant with a perennial, woody stem, or trunk, 

 wliich does not divide into branches for a certain distance above 

 the ground. Ex. elm, palm. 



b. A SHK-UB.is a plant of smaller dimensions than a tree, hav- 

 ing a perennial, woody stem, which divides into branches at or 

 near the ground, hke the alder. A slirub of diminished size is 

 termed an undershrub. Ex. whortlebeny.) 



c. An HERB is a plant \\nX\\ an annual or perennial root, pro- 



