330 THE STRUCTURE OF ECONOMIC PLANTS 



--ep 



xy 2 



Probably the correct interpretation of the nodule is to consider it 

 as a parenchymatous outgrowth of the root that is connected with 

 it by a system of vascular bundles which provide for the trans- 

 location of material. Its development results from a meristematic 

 activity of the cells which is induced by and is a response to the 

 stimulation of the invading bacteria. 



The Young Stem. — The young stem is approximately square 

 in transection with a pith consisting of large, compactly arranged, 

 parenchymatous cells that are continuous with those of the cortex 

 through the medullary rays. The major bundles are located at 



the angles of the axis -with the 

 smaller ones occupying the inter- 

 vals between. All of the bundles 

 are collateral, the phloem being 

 limited outwardly by strands of 

 elongated, thick-walled peri- 

 cyclic fibers which, with the 

 lignified xylem elements and the 

 coUenchyma, constitute the me- 

 chanical system of the stem. 

 ^^ ^ The pericyclic fibers do not form 

 a continuous zone around the 

 stele, but constitute a series of 

 strands or tangentially extended 

 sectors abutting the endodermis. 

 The sieve tubes are long and 

 narrow with simple sieve plates 

 in their end walls; and with 

 them are companion cells and 

 parenchyma. The primary xylem elements have spiral thicken- 

 ings, the secondary xylem consists of radial rows of vessels with 

 oval bordered pits, and 'between the rows are fibers and wood 

 parenchyma. (Fig. i66.) 



The cortex is limited centripetally by endodermal cells that 

 usually lack Casparian thickenings. They can be recognized by 

 their size and starch content, as well as by the occurrence of crys- 

 tals of calcium oxalate which frequently form late in ontogeny. In 

 transection, they appear as a continuous row of rectangular or 

 oval, non-chlorophyllose cells that are conspicuously larger than 

 the pericyclic cells centrad to them. Outside the endodermis 



Fig. i66. Transection of sector of young 

 stem showing vascular bundle : ca, cambium ; 

 col, coUenchyma ; en, endodermis ; ep, epi- 

 dermis ; pel, pericycle ; ph, phloem ; pi, pith ; 

 xy I, primary xylem; xy i, secondary xylem. 



