296 HOW CKOPS GEOW. 



When the cells become mature and cease growth, the 

 pits often become pores by absorption of the membrane, 

 so that the ducts thus enter into direct communication 

 with each other. 



Exogenous Plants are those whose stems contin- 

 ually enlarge in diameter by the formation of new tissue 

 near the outside of the stem. They are outside-yrowers. 

 Their seeds are usually made up of two loosely-united 

 parts, or cotyledons, wherefore they are designated 

 dicotyledonous. All the forest trees of temperate cli- 

 mates, and, among agricultural plants, the bean, pea, 

 clover, potato, beet, turnip, flax, etc., are exogens. 

 . In the exogenous stem the bundles of ducts and fibers 

 that appear in the cell-tissue are always formed just 

 within the rind. They occur at first separately, as in 

 the endogens, but, instead of being scattered throughout 

 the cell-tissue, are disposed in a circle.- As they grow, 

 they usually close up to a ring or zone of wood, which 

 incloses unaltered cell-tissue — the pith. 



As the stem enlarges, new rings of fibers may be 

 formed, but always outside the older ones. In hard 

 stems of slow growth the rings are close together and 

 chiefly consist of very firm wood-cells. In the soft stems 

 of herbs the cellular tissue preponderates, and the ducts 

 and cells of the vascular zones are delicate. The harden- 

 ing of herbaceous stems which takes place as they become 

 mature is due to the increase and induration of the 

 wood-cells and ducts. 



The circular disposition of the fibers in the exogenous 

 stem may' be readily seen in a multitude of common 

 plants. 



The potato tuber is a form of stem always accessible 

 for observation. If a potato be cut across near the stem- 

 end with a sharp knife, it is usually easy to identify upon 

 the section a ring of yascular-tissue, the general course 

 of which is parallel to the circumference of the tuber 



