PROSOPIS VELUTINA. 



27 



stems older than one year have in the cortex a varying- number of concen- 

 trically placed hard-bast rings which are broken at intervals where the 

 medullary rays penetrate the cortex. Between the rings of hard bast is to 

 be found a thin-walled parenchyma. It is the disposal of the hard bast, 

 tog-ether with the disposal of this parenchyma, that in stems 4 cm. in 

 diameter and less determines the character of the distribution of chlorophyll 

 in the inner portion of the cortex. A cross-section of a stem 4 mm. in 

 diameter shows the chlorophyll -bearing- cells of the cortex arranged in the 

 gfeneral form of a net, in which what 

 may be called the warp is the med- cTub. 



iillary rays and the woof the par- 

 enchyma, or that portion of the 

 parenchyma that separates the 

 rings of hard bast. The woof of 

 the texture in young stems occurs 

 along the inner side of the second- 

 ary hard-bast rings, but in older 

 stems, for reasons g-iven below, it 

 occupies practically the entire space 

 between the bast grroups. 



With increase in size of the stem 

 certain changes occur in the chlor- 

 ophyll apparatus which are depend- 

 ent on the dispositon of the other 

 cortical tissues. As the circum- 

 ference of the stem becomes greater 

 the g-roups of bast become farther 

 apart, while at the same time, as 

 a result of the radial pressures set 

 up, the rings are approximated 

 nearer and nearer to each other. 

 The most notice able effects of these 

 changes occur naturally in the more 

 peripheral portions of the cortex. 

 The primary medullary rays in 

 young stems extend to the hard-bast ring, and when by the'growth of the 

 stem this is broken up and its members are connected by stony tissue the 

 rays extend to the stoity tissue of this ring-. Secondary hard-bast ring-s are 

 formed within the primary one, between the segments of which pass the 

 medullary rays. With the growth of the stem the outer g-roups of sec- 

 ondary bast separate from one another, just as happened with the primary 

 bast groups, and the more peripheral rings become closely pressed together. 

 These general relations not easily described will be apparent from the 

 sketches. 



Fig. \2.—Prosopis velutina: Detail from in- 

 ner portion of cortex of stem, to show 

 structure of distal ends of medullary rays 

 and connection between outer mass of chlo- 

 rophyll {ch. b.) and the more deeply lying 

 chlorenchyma. h.b., hard bast; ch. b., cor- 

 tical chlorophyll band. 



