26 



AGRICULTURAL VARIETIES OF THE COWPEA. ETC. 



approximate pairs appear in each spike, but usually only one pai 

 develops. By preventing the formation of pods each pair of flov^ 

 may be forced to bloom successively. The pedicel of each fl 

 is very short and bears three boat-shaped, acutish bracts which are 

 early deciduous. The calyx in most varieties is conspicuously rougfcj 

 ened by transverse ridges. The calyx lobes vary much in length 

 and breadth in different varieties of each of the species. Between 

 the flowers or buds of each pair is an oblong raised cushion having 

 usually two to eight circular extra floral nectaries commonly ar 

 ranged in a single row which exude a sweet liquid that becomes 

 white when dry. An analysis of this substance by Dr. W. W 

 Garner shows it to be principally glucose. A small amount of some 

 acid, probably malic, is also present. This liquid attracts numerou.s 

 insects, including ants, honeybees, and flies. Occasionally a very 

 large cushion will have as many as 20 scattered nectaries. The 

 structure of these nectaries is well shown in Plate IV. figure 1 

 from a microscopical section prepared and photographed by Di 

 Albert Mann. The secreting cells are elongated and club shaped. 



The flowers of the cowpea open early in the morning and nearh 

 all are closed before noon. Later in the day but few open blossom 

 can be found. Each flower opens but once, wilting and collapsing 

 after blooming. The corollas vary in color from almost pure white 

 to lilac purple. In purple flowers the color is deepest on the wings. 

 ' the keels being nearly white. In nearly white flowers the last trace 

 of purple lingers on the upper edge of the wings. The back of the 

 standard is paler and often yellow or yellowish green, which gives 

 a greenish cast to the purple when viewed in front. On wilting, the 

 standard falls into the same position it occupied in the bud. Such 

 wilted flowers are nearly always yellow, which doubtless has given 

 rise to the error repeated in botanical works that the flower- are 

 " yellow " or " yellowish." In nearly all varieties a small, usually 

 W-shaped. yellow eyespot appears at the base of the standard, to the 

 base of which more or less conspicuous guide lines r extend. On 

 each side of the eyespot is a sharp vertical ridge which apparently 

 functions to raise the standard to a vertical position. 



At the base of the corolla are nectaries which secrete a small 

 amount of honey. This honey can be reached only by long-tongued 

 insects, such as bumblebees and butterflies. It would seem that a 

 heavy-bodied insect, such as a large bumblebee, could certainly push 

 down the keel enough to expose the stamens and stigma, though no 

 instances are on record where such action has been observed. At 

 Arlington Farm bumblebees have frequently been seen obtaining the 

 nectar from the flowers, but in no case under observation was the 

 stigma extruded as a result. Butterflies also can get the floral nectar 



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