We conclude that expansigeny is the basic type of aerenchyma development in roots of flowering plants and that the presence of expansigenous honeycomb aerenchyma in root cortices was fundamental to the success of the earliest flowering plants found in wetland environments.

Although flowers, leaves, and stems of the angiosperms have understandably received more attention than roots, the growing root tips, or root apical meristems (RAMs), are organs that could provide insight into angiosperm evolution. We studied RAM organization across a broad spectrum of angiosperms (45 orders and 132 families of basal angiosperms, monocots, and eudicots) to characterize angiosperm RAMs and cortex development related to RAMs. Types of RAM organization in root tips of flowering plants include open RAMs without boundaries between some tissues in the growing tip and closed RAMs with distinct boundaries between apical regions. Epidermis origin is associated with the cortex in some basal angiosperms and monocots and with the lateral rootcap in eudicots and other basal angiosperms. In most angiosperm RAMs, initials for the central region of the rootcap, or columella, are distinct from the lateral rootcap and its initials. Slightly more angiosperm families have exclusively closed RAMs than exclusively open RAMs, but many families have representatives with both open and closed RAMs. Root tips with open RAMs are generally found in angiosperm families considered sister to other families; certain open RAMs may be ancestral in angiosperms.

MEX maturation was influenced by the roots' growth medium. The MEX matures very close to the root tip in soil, but much further from the tip in hydro- and aeroponic culture. The air gap accelerated maturation of the second exodermal layer. In Iris, the type of exodermis was correlated with natural habitat suggesting that a MEX may be advantageous for drought tolerance.

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