646 



ECOLOGY 



woody, perennial stem, usually with a single primary trunk; a shruh 

 possesses a similar but smaller stem, usually mth a number of approxi- 

 mately equal shoots arising at or near the base; an herb is not con- 

 spicuously woody and usually has an annual aerial stem and often also 



a perennial underground stem. These dis- 

 tinctions, however, are arbitrary, gradations 

 between trees and shrubs being common; 

 herbs and shrubs also intergrade, especially 

 in the tropics, where all degrees of woodi- 

 ness exist and where herbaceous stems often 

 endure for two or more years. Herbs may 

 be subdivided further into forms whose stems 

 are wholly subterranean (as in many ferns), 

 forms whose stems are wholly aerial (as in 

 annuals), forms which have both aerial and 

 subterranean stems (as in most peren- 

 nials), and forms which apparently are 

 stemless (as in some rosette 

 plants). 



The display of foliage leaves 

 by steins. — Elongation and 

 branching. — From the stand- 

 point of the maximum display 

 of foliage to light and air, the 

 most significant stem features 

 are elongation, 

 branching, and 

 erectness. Elon- 

 gation is a char- 

 acteristic feature of 

 most aerial stems, 

 though it is lacking 

 in various rosette 

 plants ; growth in 

 height proceeds 

 with extreme slowness in certain other cases, particularly in the 

 cycads, some of which are said to require centuries to reach a height 

 of two or three meters. Branching is relatively slight in the pterido- 

 phytes, cycads, and monocotyls (fig. 951), reaching its culmination in 



Figs. 952, 953. — Twigs of a maple {Acer): — 952, a termi- 

 nal twig in its natural position ; note the erect terminal bud (6) 

 of the main shoot, all other buds being ascending rather than 

 erect, whether they are the terminal buds {b') of lateral shoots 

 or the lateral buds (i") of the main shoot; i, bud scales; note 

 that the terminal buds are larger than the lateral buds; 953, 

 a horizontal twig in its natural position ; note the ascending 

 curvatures of the main (m) and lateral (o) shoots. 



