DIFFERENTIATION OF TISSUES 289 



trunks (Fig. 217). This continuous addition to the vessels 

 increases the capacity of the stem for conduction, and per- 

 mits the further extension of branches and a larger display 

 of leaves. 



The annual additions to the xylem are added to the in- 

 creasing mass of wood. The older portions of the xylem 

 mass are gradually abandoned by the ascending water 

 ("sap"), often change in color, and form the heart-wood. 

 The younger portion, through which the sap is moving, is 

 the sap-wood. It is evident, however, that the annual ad- 

 ditions to the phloem are not in a position for permanency. 

 The new phloem is deposited inside of the old, and this, to- 

 gether with the new xylem, presses upon the old phloem, 

 which becomes ruptured in various ways, and rapidly or 

 very gradually peels off, being constantly renewed from 

 within. It is the protecting layers of cork (see this section 

 under Cortex), the old phloem, and the new phloem down 

 to the cambium, which constitute the so-called bark of 

 trees, a structure exceedingly complex and extremely vari- 

 able in different trees. 



The stele also frequently develops stereome tissue in the 

 form of sclerenchyma. These thick-walled fibers are often 

 closely associated with one or both of the vascular strands 

 of the bundles (Fig. 270), and lead to the old name fibro- 

 vascular bundles. 



To sum up, the stems of Dicotyledons and Conifers are 

 characterized by the development of a vascular cylinder, in 

 which the bundles are collateral and open, permitting 

 increase in diameter, extension of the branch system, and 

 a continuous increase in leaf display. 



152. Monocotyl stems. — In the stems of Monocotyledons 

 there is the same apical development and differentiation 

 (Fig. 266). The characteristic difference from the Dicotyl 

 and Conifer type, just described, is in connection with the 

 development of the vascular bundles in the stele. Instead 

 of outlining a hollow cylinder, the bundles are scattered 



