Parasitic plants must be adapted to their hosts, and hosts must have ineffective defenses. Two xeric adaptations in cacti—a broad nutrient-poor cortex and a tough hypodermis—probably are effective defenses. However, Ligaria cuneifolia overcomes these defenses with unusual methods of attack and endophytic growth, although the host (Corryocactus brevistylus) responds strongly. Ligaria cuneifolia induces conversion of thick-walled host hypodermis cells to cork cambium, creating a vulnerable area of parenchyma; it induces formation of new vascular bundles and production of wood in host leaf/bud traces and cortical bundles. The endophyte remains compact, confined near the host surface, it never reaches the host stele. Its interface with the host consists of cytoplasmic parenchyma cells and rare vessels; the endophyte appears to lack sieve tube members. Responses by C. brevistylus include: cortex cells near haustoria (but not near endophytes) produce bark, other cortex cells collapse, and although L. cuneifolia induces formation of secondary xylem in host vascular bundles leading to itself, C. brevistylus produces mostly sclerified xylem parenchyma rather than vessels. These responses are atypical of normal cactus development: L. cuneifolia induces C. brevistylus to develop in novel ways.
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