12 ELEMENTS OF BOTANY. 



explains its character perfectly. It is made up of fleshy scales (reduced 

 leaves), arranged in regular order, which gradually take on the form of 

 true leaves toward the centre and enclose the flower-stem. The onion 

 (Fig. 18) has a similar structure. 



Bulbs also present themselves in diverse forms, as the scaly and fleshy. 



One form, which is solid and of more or 

 less homogeneous structure, not made up 

 of scales, is termed a corm. That of the 

 Indian turnip (Ariscema triphyllum Tor- 

 rey) is of this character. 



Many of these underground stems 

 multiply by division or offshoots. The 

 hyacinth, for example, forms bulblets in 

 the axils of its scales, which develop into 

 new plants. And many creeping rhi- 

 zomes send up aerial stems from nearly 

 every joint. It should not be forgotten, 

 however, that all such plants grow from 

 the seed originally, in the same manner 



FIG. 18. Vertical section of an onion. , -i , -, .. . n 



as those having only aerial stems. 



The common potato furnishes a curious example of a plant with aerial 

 and underground stems, both well developed ; for the potato, termed a 

 tuber, is really a thickened portion of a subterranean stem, and each of 

 its so-called eyes a bud capable of developing into a new plant. 



Stems, like roots, are annual, biennial, or perennial. Naturally all an- 

 nual roots support only annual stems, but all subterranean stems send up 

 annual flowering stems and leaves. Perennial plants are spoken of as 

 herbaceous, suffruticose, or woody, according to whether they have 

 annual stems, those that are partly woody and do not die entirely down to 

 the ground, or those of wood sufficiently vigorous to resist the winter. 

 Woody plants under about twenty feet in height are called shrubs ; when 

 of greater height they are known as trees. This distinction is, of course, 

 somewhat arbitrary, and a given specimen may be spoken of as a shrub or 

 small tree. 



In structure stems are composed of bark, wood, and pith ; and the 

 manner in which these three are arranged, with their relations to each 

 other, serve as the basis of the division of flowering plants into two great 

 classes, namely, the exogenous and the endogenous. 



Exogenous plants have their bark, wood, and pith each distinct, as 

 shown in the cross-section of the stem of an oak (Fig. 19), in which the cen- 

 tral stellate portion is the pith, the external dark zone the bark, and the 

 intermediate part the wood. The proportions of the three vary greatly in 

 different plants, but their relative positions are always the same in exogen- 

 ous stems. 



