REACTION: The degree of acidity or alkalinity of a soil mass expressed in pH.
RECEPTACLE: The enlarged end of the stem of a flower upon which the reproductive organs of the flower are borne.
RECURVED: Of leaves or petals, curved backwards.
REFLEXED: Bent or recurved downward or backward; applied to petals, sepals, leaf-veins, etc.
REGOLITH: The unconsolidated nature of loose earth materials (soil, sand, loose rock, etc.) above the solid rock on the earth's surface.
REGULAR: Having the members of each whorl of floral organs arranged in a symmetrical shape, as in the rose and petunia.
RENIFORM: Kidney-shaped; a reniform leaf is wider than it is long, with an indentation at the base.
REPLUM: The framework that divides the various sections of certain see cases, which breaks open when mature, and that remains when the sections fall away.
RESIDUE: The amount of inorganic fertilizer, pesticide, or herbicide which remains in plants, soil, or groundwater after the growing season in which it was applied. Such residues may damage future crops, render vegetables toxic, and pollute groundwater and nearby streams.
RESMETHRIN: A synthetic pyrethrin used as an insecticide.
REST PERIOD: A period of dormancy (induced by withholding water during part of the year) that is needed by some plants in order to bring about bloom the following year.
RESUPINATE: Referring to Orchid flowers which twist 180 degrees during development, with the result that the labellum is lowermost.
RETICULATE: Net-like; resembling netting.
RETICULUM: Any plant structure with a net-like form.
RETRORSE: Turned backward or downward.
RETUSE: Notched at the tip (of a leaf or flower part).
REVERT: To return to an original state, as when plain green leaves are produced on a variegated plant.
REVOLUTE: Rolled or curled backward or downward, as the edges of the leaves of some plants.