58 THE STRUCTURE OF ECONOMIC PLANTS 



roots may also be produced abundantly. In Solanum tuberosum, 

 the rhizomes extend outward in the soil for several inches and then 

 the terminal portion begins to thicken, developing as a fleshy stor- 

 age organ, the tuber. This, like the rhizome itself, is a true stem 

 and has buds (eyes) in the axils of reduced leaves. 



Closely related to the rhizome in general habit, and often con- 

 fused with it, is the stolon or runner. Like the rhizome, this 

 arises from a lateral bud near the base of the stem axis; but it 

 remains on the surface of the ground rather than being subter- 

 ranean. It develops adventitious roots and is a means of vegetative 

 extension and propagation as illustrated in the case of certain 

 stoloniferous grasses and the strawberry. 



The corm is a type of underground stem in which the axis is 

 short, vertical, and fleshy. There is a tuft of aerial leaves above, 

 and a mass of fibrous adventitious roots below the stem, which is 

 generally sheathed by the leaf bases. Typical examples of this 

 structure occur in Gladiolus and in many of the Orchidaceae. 



ANATOMY OF THE STEM 



The stem differs from the root externally in the divergence from 

 it of lateral members, the leaves, at more or less regular intervals. 

 The places of such divergence are nodes; the intervals between them, 

 the internodes. Histologically, it is composed of tissues similar to 

 those in the root, the chief differences being the manner of dis- 

 tribution of the vascular tissue and the character and specialization 

 of the epidermal layer. 



The Herbaceous Dicotyledonous Stem. — In most herbaceous 

 dicotyledonous stems the vascular system constitutes a dictyostele, or 

 dissected siphonostele. In such a stelar type, the vascular bundles 

 which comprise it occur as a number of distinct longitudinal 

 strands, so that in transection there is a ring of vascular bundles 

 that are separated from one another radially by rays or zones of 

 parenchymatous tissue. (Fig. ix.) Great variation exists with 

 respect to the amount of parenchymatous tissue in proportion to 

 the vascular tissues, either as cortex, pith, or rays. In some cases, 

 even the vascular regions possess large quantities of parenchyma. 

 The fact that parenchymatous tissues may be present in such very 

 large proportions is one of the principal characteristics which may 

 render a plant of economic value as food for animals and man. This 

 is true because, in such tissues, large quantities of foods may be 



