Figure 3.

Parasite volume on two-host configurations. Amb to Ast, parasite stems first transplanted onto Ambrosia artimisiifolia and then trained onto Aster ericoides; Amb to Amb, parasite stems initiated then trained onto Ambrosia artimisiifolia; Ast to Amb, parasite stems initiated on Aster ericoides then trained onto Ambrosia artimisiifolia; Ast to Ast, parasite stems initiated and then trained onto Aster ericoides. These are the same parasite stems that are documented in Fig. 2. Parasite volume differed among treatments (F_{3, 29} = 4.83; P = 0.0075; an a posteriori Student–Neuman–Keuls test showed all treatments differing from one another at P < 0.05). Haustoria are functional within 4 to 5 days after contact (30). To calculate the rank order of treatments for the various hypotheses, parasite extraction of resources from hosts over the 20-day experimental period was assumed to begin 5 days after the initial transplant; three segments are defined: (i) 5 days immediately after functional establishment on the primary host, with the parasite drawing resources from that host; (ii) 5 days after the second transplant event, but before functional establishment on the second host, with the parasite still drawing resources only from the primary host; (iii) 5 days after establishment on the second host, with the parasite drawing resources from both host individuals. There were no significant differences in parasite coil lengths among the four treatments (F_{3, 31} = 1.69; P = 0.1891).