STRUCTURE OF THE STEM 



71 



87. The Early History of the Stem. — In the earliest 

 stages of the growth of the stem it consists entirely of 

 thin-walled and rapidly dividing cells. Soon, however, 

 the various kinds of tissue which 

 are found in the full-grown stem 

 begin to appear. 



In Fig. 43 the process is shown 

 as it occurs in the castor bean. 

 At m, in B, is the central column 

 of pith surrounded by eight fibro- 

 vascular bundles, /w, each of 

 which contains a number of ducts 

 arranged in a pretty regular man- 

 ner and surrounded by the fore- 

 runners of the true wood-cells. 



In C the section shows a con- 

 siderable advance in growth: the 

 fibro-vascular bundles are larger 

 and are now connected by a rap- 

 idly growing layer of tissue, ch. 



As growth continues this layer 

 becomes the cambium layer, com- 

 posed of thin-walled and rapidly 

 dividing cells, as shown in Fig. 45. 



88. Secondary Growth. — From 

 the inside of the cambium layer 

 the wood-cells and duets of the 

 mature stem are produced, while 

 from its outer circumference the new layers of the bark 

 proceed. From this mode of increase the stems of dicoty- 

 ledonous plants are called exogenous, that is, outside- 

 growing. The presence of the cambium layer on the 



Fig. 43. Transverse Section 

 through the Hypocotyl of 

 the Castor-Oil Plant at Vari- 

 ous Stages. (Considerably 

 magnified.) 



A, after the root has just ap- 

 peared outside the testa of 

 the seed ; B, after the caulicle 

 is nearly an inch long ; C, at 

 the end of germination; r, 

 cortex (undeveloped bark) ; 

 m, pith ; st, medullary rays ; 

 fv, fibro-vascular bundles; 

 cb, layer of tissue which is to 

 develop into cambium. 



