Hydathodes in Physocarpus (Rosaceae: Spiraeoideae)

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Abstract
This is the first anatomical study of hydathodes from subfamily Spiraeoideae. Fresh leaves of Physocarpus opulifolius were cleared, or processed for paraffin and plastic sections and scanning electron microscopy. Each marginal tooth apex bears an achlorophyllous hydathode, which is visible adaxially as a smooth epidermal pad studded with 15–25 small, sunken "water pores" usually covered by an unbroken cuticle. Ordinary stomata are larger, raised, and abaxial. Internally, an epithem of small, loosely arranged cells extends from the adaxial epidermis to the laterally broadened, single vein ending. Bundle and epithem are bounded by the bundle sheath, which extends to the epidermis at the periphery of the pad. Guttation neither was seen naturally nor could it be induced. Cleared leaves of herbarium specimens of the six Physocarpus species showed all degrees of hydathode reduction to complete absence.