84 OUTLINES OF PLANT LIFE. 



to envelop the stem completely and hide it from view. This 

 is well seen in the scale-like leaves of such plants as the pines 



(fig. 63), cedars, and arbor vitas 

 (fig. 71). Or, certain of the 

 internodes may elongate, while 

 others remain undeveloped. 

 For example, in the shepherd's- 

 purse, the first internodes re- 

 main short, so that the lower 

 leaves are crowded into a tuft 

 or rosette; the following inter- 

 nodes are elongated, the corre- 

 sponding leaves being scattered 



Fig. 71.— A shoot of arbor vitse or white ^4. «„„„i„.. ;«+rt«.«1.^. „,l.:i» 

 cedar, showing scale leaves covering at regular intervals; whllc, 

 stem. NatumTsize.-AfterKemer. gtill higher, the intCmodeS are 



again shortened and the leaves brought into close clusters in 

 the flowers. 



105. A section of the stem commonly presents an irregularly circular 

 outline (fig. 72). Occasionally the surface of the stem is fluted or chan- 

 neled, and, if these grooves or channels be few, and the corresponding 

 angles prominent, the section of the stem is polygonal, with three, four, 

 five, six, or more sides (fig. 131). 



106. Habit. — As to habit, stems are commonly erect 

 when enough mechanical tissue is developed to render them, 

 sufficiently rigid to carry not only their own weight, but that 

 of the leaves and other members attached to them. Other 

 stems lie flat upon the ground, to which they may or may 

 not attach themselves by the development of secondary roots. 

 Between these prostrate, or creeping, stems and the erect form 

 every conceivable position exists. The direction of growth 

 is determined largely by the relation of the plant to gravity 

 and light as stimuli. (See W 243, 245.) Other stems rise 

 into the air, not by their own rigidity, but by the develop- 

 ment of special members for climbing purposes, such as 



