...the calix is a partial involucre or involucret Situated at the base of the footstalk of each flower on the peduncle; it is long thin and begins to decline as soon as the corrolla expands.
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His expression "the calix is a partial involucret situated at the base of the footstalk" is an incorrect understanding of floral parts. What Lewis is referring to here is the subtending bract at the base of each pedicel. These bracts are associated with the bud and act as a protective layer over the bud as the inflorescence is elongating and the buds are maturing. As the buds mature to the point of anthesis (flowering, or the opening of a flower), the bracts wither and are torn apart by the expanding bud. By the time an individual flower opens, the bracts are usually decurrent (bent downwardly) and well away from both the flower and the pedicel.
Glossarycalyx--"All of the sepals of a flower, collectively;" written "cali"X by Lewis and others of his day.
sepal--"A member of the outermost set of floral leaves, typically green or greenish and more or less leafy in texture."
involucre -- "A set of bracts beneath an inflorescence; more generally, any set of structures which surround the base of another structure."
peduncle--"The stalk of an inflorescence or of a solitary flower."
subtend--"To be directly below or close to."
decline -- "Curved downward."
bract -- "A specialized leaf, from the axil of which a flower or flower stalk arises; more loosely, any more or less reduced or modified leaf associated with a flower or an inflorescence, but not a part of the flower itself."
axil--"The point of an angle formed by the leaf or petiole with [the stem]."
Supported in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee.