The Inner Structure of the Plantlet. 55 



tion between bark and wood; the stem enlarges for a 

 time by growth throughout its whole diameter, after 

 which it ceases to expand. 



In plants having two or more cotyledons, however, 

 additions to the bark cells are constantly being made 

 during the growing season on the outside of the cam- 

 bium layer, as are additions to the wood cells on the in- 

 side 'of it (Fig. 22). It follows that growth of the bark 

 takes place on its inner surface and growth of the wood 

 takes place on its outer surface. This explains the ver- 

 tically-furrowed appearance of the bark of old trees 

 which is being constantly split during the growing 

 season by the forming layer within. It also explains 

 the ringed appearance of a cross-section of a woody 

 stem. A new ring of wood is formed each season on 

 the outside of that previously formed, and the line sep- 

 arating the rings marks the point where growth in 

 autumn ceased and was renewed the following spring. 

 The age of a given part of the stem of a woody plant 

 is approximately indicated by the number of its wood 

 rings.* 



71. The Vital Part of Woody Stems in plants having 

 more than one cotyledon (45) is limited to a rather 

 thin layer of bark and wood, of which the cambium 

 (68) forms the center. The cells of the so-called heart- 

 wood and those of the dry and furrowed outer bark, 

 have lost their protoplasm, and hence are no longer 

 alive, though they serve a useful purpose in adding 

 strength and protection to the vital layer. The heart- 



* More .than one wood -ring is sometimes formed in a season. If 

 growth ceases during the summer from severe drought or other cause, 

 and is renewed the same season, an extra ring is formed. 



