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 g^reen 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 

 gfreen. 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 

 amoimt 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: 



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 (l) 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. 



