54 Principles of Plant Culture. 



the two portions of cambium may unite by growth, a 

 fact of great importance in horticulture since it renders 

 grafting possible (383). Plants having no cambium layer 

 (71) cannot be grafted, because their steins have no 

 layer of dividing cells the only cells that unite by 

 growth. 



71. How Stems Increase in Diameter. There is no 

 cambium layer in plants having but one cotyledon (46 ), 

 of which Indian corn, the grasses and the palms are ex- 

 amples. In such plants there is no clear separation be- 

 tween 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, ad- 

 ditions to the bark cells are constantly being made dur- 

 ing the growing season on the outside of the cambium 

 layer, as are additions to the wood cells on the inside 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 vertically - 

 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 ap- 

 pearance 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 separating 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 in- 

 dicated by the number of its wood rings.* 



* 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 re- 

 newed the same season, an extra ring is formed. 



