PROSOPIS VELUTINA. 29 
In the early part of the first year of growth the branch is smooth and of 
a dark purple color; later in the season it becomes green and is flecked with 
minute purple or red dots and is somewhat rough to the touch. During 
the following season and for an undetermined period afterwards it remains 
green. Finally this is replaced by a gray surface which also is slightly 
rough and which persists for several years. Branches 8 cm. in diameter 
may have this appearance. The gray exterior is in turn replaced in old 
stems by a rough bark black in color. 
- ‘The color of the youngest stem is due to red and blue pigment in the 
epidermis, and the texture of the surface to the unbroken cuticle. As the 
stem becomes older, phellogen arises in the epidermis, which forms primary 
periderm and primary phelloderm. Ruptures appear in the cuticle, which 
become pronounced and allow the chlorophyll to be seen through the corky 
tissue. As the stem becomes larger the amount of cork increases, the 
amount of phelloderm especially becomes greater, and at length entirely 
conceals the underlying chlorophyll. This condition lasts a long time and 
constitutes the third stage, as presented above. Finally, in still older stems 
a secondary phellogen is organized deeper in the cortex than the chlorophyll 
band and separates this tissue to its ruin from the remainder of the stem. 
After the formation of the secondary phellogen the stem does not as a con- 
sequence contain more chlorophyll. 
The following measurements were taken: 
istan Width of | Depth of 
otbranch. | from tip. |chlorophyu Morphy! chlorophyll 
mm em. mm. B tad 
1.3 3 0.65 32 32 
1.6 18 0.80 32 48 
2.25 33 1.25 64 64 
3.3 48 1.65 64 48 
4-5 63 2.25 64 48 
5 78 2.32 64 48 
7 93 3-30 64 48 
9 108 3-30 106 32 
15 123 | saeeee 100 48 
SALIX NIGRA. 
Salix occurs in some abundance along the banks of the bed of the Santa 
Cruz River. Some of the trees may attain a height of 15 m. or more. 
A section of a branch 3.5 mm. in diameter 30 cm. from the tip shows the 
following general relations of the tissues: Beginning with the periphery 
there is (1) a protective portion about four cells deep, which does not 
contain coloring matter, and a protective portion beneath this about two 
cells in thickness that is pigmented; (2) a parenchymatous tissue which is 
chlorophyllaceous; (3) groups of hard bast and inner ground-tissue and 
cambium; and (4) finally, the woody cylinder. 
