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, 
together 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 
general form of a net, in which what 
may be called the warp is the med- 
ullary rays and the woof the par- 
enchyma, or that portion of the 
parenchyma that separates the 
tings 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 oider 
stems, for reasons given below, it 
occupies practically the entire space 
between the bast groups. 
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 groups of bast become farther 
apart, while at the same time, as 
a result of the radial pressures set 
up, the rings are approximated 
Fig. 12.—Prosopis velutina: Detail from in- 
nearer and nearer to each other. 
The most notice able effécts of these 
changes occur naturally in the more 
peripheral portions of the cortex. 
ner portion of cortex of stem, to show 
structure of distal ends of medullary rays 
and connection between outer mass of chlo- 
rophyll (ch. 5.) and the more deeply lying 
chlorenchyma. .4., hard bast; ch. 6., cor- 
The primary medullary rays in tical chlorophyll band. 
young stems extend tothe 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 stony tissue of this ring. Secondary hard-bast rings are 
formed within the primary one, between the segments of which pass the 
medullary rays. With the growth of the stem the outer groups 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. 
