Phacelia (Hydrophyllaceae) Juss. is a primarily western North American genus of over 200 annual and perennial species. Historically, calyces in Phacelia subsect. Humiles have provided important taxonomic characters with published descriptions giving mature calyx sizes as ranges rather than a specific number, and with little attention paid to the intra- or interspecific biological significance of this variation. This paper tests the hypothesis that Phacelia sepals are phenotypically plastic organs and that their post-accrescent size in Phacelia subsect. Humiles is related to seed production in associated fruits. Examination of ten taxa within Phacelia subsect. Humiles revealed that calyx size variation within a species was positively correlated with seed weight per capsule for nine of the ten taxa examined. Experimental data showed the correlation was not due to allometry. In defoliation experiments plants grown with all leaves removed post-germination were able to flower and produce seed solely through cotyledon, stem, and calyx photosynthesis, with the sepals expanding in size well beyond normal parameters. This and other data demonstrated that the sepals have developmental plasticity, are important photosynthetic organs, that the seed-weight/sepal-size correlation represents a functionally important source-sink relationship, and that pedicel length and inflorescence density are frequently related to calyx morphological characteristics.