Isolation frequency for endophytes from photosynthetic stems of Sonoran Desert plants reflects a site × species × sampling month × exposure interaction (see Table 2). Data indicate the percent of tissue segments from stems of Larrea tridentata, Parkinsonia microphylla, and Simmondsia chinensis that yielded endophytes in culture, as a function of sampling site (A, King Canyon; B, Red Hills), sampling month (October vs. February), and exposure (flat, exposed; south- or west-facing, highly exposed; or north-facing, protected).