38 
Journal of Agricultural Research 
Vol. II. No. t 
cavity occupies almost the entire width of the cell. The walls are of 
exceedingly dense cellulose. An outside cuticular sheath in rare cases is 
found covering the upper or outer surface of the palisade layer, but in most 
instances it is lacking (PI. VI, fig. i, cu ), and the narrowed cavity of the 
cell seems to either reach or almost reach the outer surface of the seed 
coat. That there often is an actual aperture at the upper end of the 
cell is easily demonstrated; for when stains or colored reagents are used, 
it is easy to trace the rapid inflow of the liquid through these narrow, 
threadlike extensions of the cell cavity downward into the larger area 
at the lower end. Air bubbles are also seen to be pushed forward by the 
inflowing liquid and to pass downward into the large basal cavity. The 
bearing of this fact on the absorption of moisture necessary for the 
germinating of the seed is evident. 
The normal form of the palisade cells in some varieties is strangely 
modified, the cell walls being very irregular. The taper of the cell cavity 
in such cases is imperfect, and after suddenly narrowing from the wide 
basal portion to a mere thread it again expands toward the outer end 
into a sort of mushroom-shaped enlargement. The cells themselves 
are also greatly contorted in outline and are sometimes spirally twisted 
upon their long axis, so that a true longitudinal section of these cells, such 
as is usually obtained in a transverse section of the seed coat, is quite 
impossible. Sometimes they are more or less intertwined. These dis¬ 
tortions, as will be pointed out, are associated with certain color schemes 
and are quite characteristic of certain varieties. 
There are two classes of pigment found in the palisade cells: First, a 
melanin-like pigment, identical in all its reactions and similar in its color 
to the pigment referred to in the lower of the three layers—namely, the 
basal-color layer. In some instances this is present in all the palisade 
cells, thereby supplementing and intensifying the basal color of the seed 
coat. In most cases it is confined to small groups of cells interspersed 
among larger or smaller areas of the palisade layer destitute of this pig¬ 
ment. According as this pigmentation is uniform or irregular in its 
deposit the basal color of the seed coat is uniform or mottled. 
More frequent than this melanin-like pigment in the palisade layer are 
various anthocyanin pigments. These also may fill uniformly all the 
palisade cells or may be variously grouped and interspersed with colorless 
cells, thereby giving rise to the very diverse color schemes characteristic 
of different varieties of cowpeas. The anthocyanin pigments are prac¬ 
tically of two kinds: First, an acid-reacting anthocyanin, ranging in color 
from a decided rose red to a strong purple; and, second, an alkaline-react¬ 
ing anthocyanin, uniformly of a deep indigo blue, but which in mass often 
appears as dead black. 
In many instances only one of these phases of anthocyanin pigment is 
discoverable in the seed coat of a given variety; and according as it fills 
