The wind tunnel used in these experiments had three plexiglass components (Fig. 1). Component A consisted of three linked sections, the first, a 72 mm diameter, 30 mm long cylinder, containing an axial fan connected to a tension regulator, allowing regulation of wind speed. The second section was a 100 mm long funnel-shaped tube, tapering from 72 to 30 mm. The third section, a 30-mm diameter, 150 mm long cylinder, was pierced by a hole through which a hot-wire anemometer (TESTO 491, TESTOTERM, Forbach, France; precision of 2.5, 4, and 5% for wind speed ranges of 0–3, 3–10, and 10–60 m s−1, respectively) could be inserted to measure the wind speed near the sporulating leaf disc. Component B, a 30 mm diameter, 100 mm long cylinder, was the sample chamber. On the inner edge of this, next to component A, a removable ring (16 or 30 mm interior diameter, 20 mm long) could be inserted (Fig. 1, shaded). The ring was used to insert two types of sample stands, one being a horizontal, vaseline-coated plastic plate, on which the sporulating leaf disc was placed and remained fixed, and the other a vertical flexible needle onto which the sporulating leaf disc was pinned and was thus free to vibrate when exposed to wind. This second stand was used to study the effects of leaf movement and rain tap. The final component (C) was an 80 mm long funnel-shaped tube, tapering to a 13.3 mm diameter orifice. To simulate wind speeds lower than 4.4 m s−1, the wind tunnel was used with all three components, and the 30 mm diameter removable ring was installed. To simulate higher wind speeds, (up to 17 m s−1), the internal diameter of the wind tunnel had to be reduced. Two adjacent rings (100 mm long and 16 mm diameter each) were then inserted in the components A and B, and the 16 mm diameter removable ring was used. Component C was removed. Given the internal diameter of the wind tunnel, and according to Poiseuille's law, the flow was laminar in all the experiments.
A hole, 2 mm in diameter, drilled above the sample stage in component B, allowed access for investigating the rain tap effect. To simulate the impacts of raindrops, lead beads were dropped on the sporulating leaf disc from the top of a vertical hollow cylinder fixed above the hole.
Humidity control was obtained by mixing dry and saturated air, passed through a drier and humidifier, respectively (Fig. 2). The drier consisted of a 15 cm diameter, 25 cm long plastic cylinder filled with silica gel. The humidifier was a similar cylinder filled with soaked corrugated paper. A 75 W lamp 5 cm away provided heat to increase water evaporation from the paper. The drier and humidifier were extended by 30 cm long parallel plastic tubes, which merged into a main tube (20 cm long) fixed to the wind tunnel, next to the fan. A hole was drilled in the main tube to insert a hygrometer sensor (model H 6.2 sensor connected to a portable hygrometer indicator HUMICOR 1100, CORECI, Lyon, France; precision of 2% within a 20–100% range). Each parallel tube was equipped with a valve. Dry and nearly saturated air could be combined in different proportions by operating the two valves. The r.h. could be lowered or raised within the range of 20%-100% within 3 min. When the humidity device was connected to the wind tunnel, the maximum wind speed achievable, measured with the anemometer, was 3.6 m s−1 for low r.h. (20%) and 17 m s−1 for high r.h. (> 90%).