Development of bicellular foliar secretory cavities in white snakeroot, Eupatorium rugosum (Asteraceae)

Bicellular secretory cavities in Eupatorium rugosum occur only in foliar mesophyll, distributed uniformly from leaf tip to base, with a mean density of 450 per mm2. They are absent from petiole and all other vegetative and floral organs. Each cavity contains an oily droplet, which is surrounded by two cup-shaped cells. An initial cell divides into two equal cavity cells with their shared wall always perpendicular to the epidermis. After vacuoles form, each protoplast retracts from the other and deposits a new, callosic wall around the cavity, and a thicker callose deposit on the remaining shared walls outside of the cavity. The original shared wall remains intact across the cavity until maturity. It is pulled taut by cavity cell growth and restricts further expansion except in one paradermal direction. Later, this shared wall disappears from the cavity. An oily fluid of unknown composition is secreted into the enlarging cavity. Because bicellular cavities develop neither lysigenously nor in true schizogenous fashion, we propose the term "pseudoschizogeny" for this type of development. Unusual, or perhaps unique, features of this cavity: bicellular condition, protoplast retraction from the common shared wall, and deposition of a callose wall. Preliminary results of a survey of Eupatorium show that bicellular cavities are uncommon but widely distributed geographically.