SECONDARY GROWTH 



65 



B 



In Fig. 66 the process is shown as it occurs in the castor 

 bean. At m, in B, is the central column of pith surrounded 

 hy eight nbro-vascular bundles, fv, each of which contains a 

 number of vessels arranged in a somewhat regular manner and 

 surrounded by the forerunners of the 

 true wood cells. 



In C the section shows a consider- 

 able advance in growth: the fibro- 

 vascular bundles are larger and are 

 now connected by a rapidly growing 

 layer of formative tissue c. 



As growth continues, this layer 

 becomes the cambium layer, com- 

 posed of thin-walled and rapidly 

 dividing cells, as shown in Figs. 

 67 and 68. 



80. Secondary growth. From the 

 inside of the cambium layer the wood 

 cells and ducts of the mature stem 

 are produced, while from its outer 

 circumference the new layers of the 

 bark proceed. From this mode of in- 

 crease the stems of dicotyledonous 

 plants are called exogenous, that is, 

 outside 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 alder, 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. It is also, as might be 

 supposed, very rich in proteids. 



FIG. 66. Transverse section 

 through the hypocotyl of 

 the castor-oil plant at vari- 

 ous stages 



A, after the root has just ap- 

 peared outside the testa of the 

 seed ; B, after the hypocotyl 

 is nearly an inch long ; C, at 

 the end of germination; r, 

 cortex (undeveloped bark) ; 

 m, pith ; st, medullary rays ; 

 fv, fibre-vascular bundles ; 

 c, layer of tissue which is to 

 develop into cambium. Con- 

 siderably magnified. After 

 Sachs 



