Seed Discharge in Arceuthobium: A Photographic Study
Photographs, made at a speed of 5 microseconds, of the ejection of the Arceuthobium seed from its fruit, show that tumbling in the vertical plane begins soon after discharge and that most seeds have turned 90 degrees when they have traveled about 6 centimeters. As the seeds leave the fruit they have a viscous sheath around the trailing end that disintegrates after the seeds have traveled about 2 to 3 centimeters. This viscous material does not act as a rudder as has been postulated.
Seed Dispersal Velocity in Four Dwarfmistletoes
By means of a high-speed photographic technique, the initial velocities of seeds were studied as they were expelled from the fruits of four Colorado dwarfmistletoes: Arceuthobium douglasii, A. campylopodum f. cyanocarpum, A. vaginatum f. cryptopodum, and A. americanum. Velocities of the seeds of the latter two species averaged 2600 centimeters per second and were significantly greater than those of the first two, which averaged 2200 centimeters per second. The initial velocity of 526 seeds of the four dwarfmistletoes averaged 2400 centimeters per second.
Testosterone levels, handedness and sex ratio at birth.
Geschwind has hypothesized that levels of androgens in utero affect postnatal laterality preferences, high levels being associated with left-handedness of offspring. I have hypothesized that high parental androgen levels at the time of conception are associated with subsequent male births. However, left-handed parents seem to produce an excess of female, not male, offspring. This finding could be reconciled with the two hypotheses if individuals who were exposed to high levels of androgens in utero had low circulating levels in adulthood. Some evidence for this loophole is reviewed here. Lyster & Lloyd (1987, J. theor. Biol. 126, 125) have reported that (consistent with the hypotheses) index left-handed men have an excess of brothers; but, contrary to the hypotheses, index left-handed women have an excess of sisters. Accordingly these authors proposed that left-handedness in females is associated not with high levels of androgen in utero, but with a "sensitivity" to it. If such sensitivity were associated with low androgen levels, it might explain an excess of sisters among the sibs of left-handed women. It is shown here that evidence in agreement with this hypothesis (though not discriminating between it and Geschwind's) suggests that the sex ratios of offspring of LH male x RH female and of RH male x LH female matings, are both low.