THE S TEM. 



97 



therefore, crowded. They may be so crowded as to completely 



envelop the stem and hide it 



from view. This is well seen 



in the scale-like leaves of such 



plants as the pines (fig. loi), 



cedars, and arbor vitae (fig. log). 



Or, certain of the internodes 



may elongate, while others 



remain undeveloped. For 



example, in the shepherd's- 



purse, the first internodes 



remain short, so that the lower 



lea\-es are crowded into a tut"t ^ic. 109.— a shoot of arbor \it3; or white 



cedar, snouing scale lea\es covering 

 or rosette; the following inter- ^t^™- >-'alural size.— After Kemer. 



nodes are elongated, the corresponding leaves being scattered 

 at regular intervals : while, still higher, the internodes are 

 again shortened and the leaves brought into close clusters in 

 the flowers. 



120. The consistence of the stem depends upon the relative 

 amount of mechanical tissues which it contains. Stems may 

 be designated as woody, solid, or fleshy, terms which need no 

 further definition. 



121. The shape of the stem varies e.xtremely in difterent 

 plants. Verv commonly the stem as a whole is looked upon 

 as cylindrical, but, if carefully considered, it will be seen that 

 the diameters of successive internodes at first become gradually 

 greater, and, after maintaining this maximum for a time, grow 

 gradually less. The stem is, therefore, a cylinder with more 

 or less conical ends. If the attainment of the maximum 

 diameter is sudden, and the diminution similarly sudden, the 

 resulting stem will ha^■e the shape of a double cone. The 

 modification of such a form into the spherical is not difiicult 

 to imagine. Striking illustrations of these extreme forms are 

 to be found among the cactuses (hg. no). 



