36 
Journal of Agricultural Research 
Vol. II, No. i 
zontal. They are considerably longer than broad and are, as a rule, so 
arranged that their longer horizontal axis is at right angles to the slightly 
longer horizontal axis of the cells of the second layer (fig. i, c). One of 
the advantages of this arrangement may be to give tensile strength to 
the seed coat. 
The middle layer of relatively cubical cells plays no r 61 e whatever in 
the pigmentation of the cowpea. Its cells are practically empty, only 
such residue of organic matter being present as would necessarily be 
found there. It is possible that there is some effect upon the coloration 
due to the included air which fills these cells and which in fresh-mounted 
sections always appears like a black band through the section; but, as 
the resulting color of the seed coat is made up from the different factors 
taken vertically, this single-celled empty layer between the palisade and 
the basal layers must have extremely little influence upon the color. 
PIGMENTATION OF THE BASAL-COLOR LAYER 
The third or inner layer is more or less filled with a pigment, which is 
the same in all the cowpeas examined, and for that reason the writer 
calls this the basal-color layer. The pigment is a melanin-like substance, 1 
ranging from a pale-straw color to a deep orange or heavy buff. As a 
rule, it is massed in granular particles in the lower part of the layer. 
In some cases the upper cells contain the larger amount of pigment, and 
in a few instances it is evenly distributed throughout all the cells of 
the basal layer. In some cowpeas, and especially in those that have a 
heavy basal color, the pigment completely fills a large part of the cells 
and is then seen to be crystalline. In such cases the color is a deep 
orange or sometimes even a copper red. No trace in any instance was 
found of any other pigment in this basal layer. Anthocyanin tests 
failed in every case to give a reaction. 
No attempt is made in this paper to discuss the cell contents of the 
seed coat, outside of those substances which are directly concerned in 
producing the color schemes found to exist in ripe cowpeas. How the 
various pigments arise in the growing cells and what are the mechanical 
principles back of their predetermined distribution in the different 
varieties are questions of great cytological interest, but not important 
for the subject in hand. It may, however, be worth while to mention 
here one substance which is very generally associated with the different 
pigments—namely, tannin. Tests with such reagents as ferric chlorid, 
ferric acetate, potassium bromate, osmic acid followed by hydrogen 
1 The applying of the term *'melanin” to any plant pigment has been criticized. (See Gortner, R. A. 
The misuse of the term "melanin.” Science, n.s.,v. 36, no. 915, p. 52-53. 1912.) Although Mr. Gortner, so 
far as the writer knows, has not sufficient ground to warrant his exclusion of this term from plant nomen¬ 
clature, seeing that the statement that it never occurs in plants is unproved, the writer agrees that its use 
here is open to criticism, and has therefore substituted "a melanin-like pigment" because no advantage 
can be found in employing Osborn’s term " humin, ’ ’ favored by Gortner, the boundaries of this term being 
at present as vague and unsettled as those of mela n i n . 
