96 FOUNDATIONS OF BOTANY 



one-year-old Aristolochia-stem (Fig. 56) shows a decided 

 similarity between the two. In both cases we have the 

 central pith, the regularly grouped bundles, and cambium 

 (or in Fig. 68, C, a tissue which will grow into cambium), 

 — part of it in the bundles and part of it between them. 



In the young monocotyledonous stem the grouping of 

 the bundles is less regular than that just explained. This 

 is shown by Fig. 52. A much more important difference 

 consists in the fact that the monocotyledonous stem has 

 usually no permanent living cambium ring. Annual dicoty- 

 ledons, however, are also destitute of permanent cambium. 



108. Secondary Growth. — From the inside of the cam- 

 bium layer the wood-cells and dusts of the mature stem 

 are produced, while from its outer circumference proceed 

 the new layers of the inner bark, composed largely of sieve- 

 cells and hard bast. From this mode of increase the stems 

 of dicotyledonous plants are called exogenous, that is, out- 

 side-growing. The presence of the cambium layer on the 

 outside of the wood in early spring is a fact well known 

 to the schoolboy, who pounds the cylinder cut from an 

 elder, willow, or hickory branch until the bark will slip 

 off and so enable him to make a whistle. The sweet taste 

 of this pulpy layer, as found in the white pine, the slippery 

 elm, and the basswood, is a familiar evidence of the 

 nourishment which the cambium layer contains. 



With the increase of the fibro-vascular bundles of the 

 wood the space between them, which appears relatively 

 large in Fig. 68, becomes less and less, and the pith, which 

 at first extended freely out toward the circumference of 

 the stem, is at length only represented by thin plates, the 

 medullary rays. 



