10 



ELEMENTS OF BOTANY. 



In annual exogenous i)lants, though the relative positions of the bark, 

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

 the woody tissue is arranged ^^ ith less unif orniity, often in wedge-shaped 

 bundles, and the bark ha.s no distinctly corky layer. 



Endogenous plants have no central mass of pith entirely free from 

 woody libres, 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 j^criod 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 botli longitudinal 

 and cross-section. There will ap- 

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

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

 this cannot be sei^aratcd as in the case of an exogenous plant, since it is 

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

 pact structure within. 



Endogenous plants very commonly have simple stems, though branch- 

 ing ones are not unusual. In teiiiperate regions thej* are chiefly small 

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

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

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



Fig. 2.J, — Cross-se.Uuu of tho stem of a palm. Tho 

 outer circle is conilcnsud wrjody tissue; within, tho 

 dots represent bundles of woody tissue inibeilded in jtith. 



FUNCTIONS OF THE STEM AND BRANCHES. 



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

 means of comnumicatiou 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 towai'd 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- 

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



