In what is often cited as a case of convergent evolution, several species in the African genus Aloe (Aloaceae) are similar to North American agaves and yuccas (Agavaceae) with respect to morphology, physiology, and habitat preference. A belowground feature common to 21 of the 23 species examined in the Agavaceae is a contractile root zone near the shoot base. In taller species such as Joshua tree (Yucca brevifolia), roots were contractile along much of their length. External evidence of root contraction is transverse wrinkling of the epidermis and exodermis (outermost layers); internal evidence is radial expansion of cortical cells and sinuous compression of xylem vessels. Root contraction pulled shoots deeper in the soil; for example, in an unheated glasshouse, seedlings of Agave parryi were pulled down an average of 4.5 cm in four months, and two-year old plants of Hesperoyucca whipplei and Y. brevifolia were pulled down 3.9 cm and 5.1 cm respectively in twelve months. The contractile, basal zone of agave and yucca roots had significantly greater hydraulic conductance than the younger non-contractile root regions and had anatomical features associated with increased water permeability, such as reduced suberization in the endodermis and prolonged viability of cortical cells. In contrast, no evidence for root contraction was observed in the seven species of Aloe examined, nor was hydraulic conductance greater for basal root regions than for other root regions. The presence or absence of contractile roots may be related to differences in phylogeny and/or habitat between the North American and African taxa.
