16 



ELEMENTS OF BOTANY. 



In annual exogenous plants, though the relative positions of the bark, 

 wood, and pith are the same as described above, the pith is relatively large, 

 the woody tissue is arranged with less uniformity, often in wedge-shaped 

 bundles, and the bark has no distinctly corky layer. 



Endogenous plants have no central mass of pith entirely free from 

 woody fibres, no concentric rings of wood, and no separable bark. Their 



wood is in bundles or fibres inter- 

 mingled with the cellular tissue, 

 and new bundles are continually 

 being formed in the midst of the 

 old during the period of growth. 

 Toward the external surface of 

 the stem the wood becomes con- 

 densed and hard, and serves, in- 

 stead of bark, as a protective cov- 

 ering to the more tender parts 

 within (Fig. 25). 



Take, as an illustration famil- 

 iar to all, a stalk of Indian corn, 

 and view it in both longitudinal 

 and cross-section. There will ap- 

 pear an abundance of cellular tissue (pith), but with numerous strong 

 woody fibres interspersed ; and though it appears to be covered with bark, 

 this cannot be separated as in the case of an exogenous plant, since it is 

 only condensed woody tissue and shades off gradually into the less com- 

 pact structure within. 



Endogenous plants very commonly have simple stems, though branch- 

 ing ones are not unusual. In temperate regions they are chiefly small 

 plants the grasses, sedges, and cat-tails are familiar examples but from 

 their immense number form a very important part of the vegetation. In 

 the tropics many of them, chiefly palms, attain the stature of tall trees. 



FIG. 25. Cross-section of the stem of a palm. The 

 outer circle is condensed woody tissue ; within, the 

 dots represent bundles of woody tissue imbedded in pith. 



FUNCTIONS OF THE STEM AND BRANCHES. 



The most important function of the stem and branches is to serve as a 

 means of communication between the roots, leaves, and reproductive or- 

 gans, for to this one function all others are subsidiary. 



We may safely assume that a plant's whole energies are bent toward the 

 reproduction of its species. When this is accomplished the annual and 

 the biennial die ; the perennial goes a step farther and prepares for a 

 repetition of the process the next year, and then it ceases labor for the 

 season. 



The stem and branches, then, supply the channels by which the nour- 

 ishment collected by the roots is transmitted to the leaves for elaboration 



