THE GBOWING PLANT 47 



of cambium from one part of a plant is closely applied to 

 the cambium of another part of the same plant, or of 

 another closely related plant, the two portions of cambium 

 may unite by growth, a fact of great importance in horti- 

 culture since it renders grafting possible (383). Plants 

 having no cambium layer (70) cannot, as a rule, be 

 grafted, because their stems have no layer of dividing 

 cells — the only cells that unite by growth. 



70. How stems increase in diameter. — There is no 

 cambium layer in plants having but one cotyledon (45), 

 of which Indian corn, the grasses and palms are examples. 

 In such plants there is no clear separation between bark 

 and wood ; the stem enlarges for a time by growth through- 

 out 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 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 indicated 

 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 



