52 Principles of Plant Culture. 
within the cambium line corresponds to the wood of woody 
stems, and that without it corresponds to the bark. 
70. Portions of Cambium from different plants may 
Unite by Growth. Ifa section 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 horticulture since it renders grafting 
(383) possible. Plants having no cambium layer (71) can- 
not be grafted, because their stems 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 examples. 
In such plants there is no clear separation 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, addi- 
tions to the bark cells are constantly being made, during the 
growing season, on the outside of the cambium layer, and 
additions to the wood cells, on the inside of it, (Fig. 20). It 
follows that growth of the bark takes place on its inner sur- 
face, and growth of the wood takes place on its outer sur- 
face. This explains the vertically-furrowed appearance of 
the bark of old trees, which is constantly being split open, 
during the growing season, by the newly 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. 
