Phloeonomus looks at Dioecy
Dioecious Proteaceae - are there always more males than females in the population?
Roy Lubke, Grahamstown
William Bond, who has investigated the demography of Leucadendron, replies:
Unbalanced sex ratios are common in dioecious species, including Leucadendron. Males are usually most abundant, but sometimes the reverse occurs. If the class had observed sex ratios at different positions along a moisture gradient, such as from a valley bottom to the drier slopes, they would probably have seen a switch in dominance. Females are usually most common in the least stressful sites, such as deeper or moister soils, with a switch to males as conditions become harsher.
The usual explanation for these changing sex ratios is the different resource requirements of the sexes, arising from their different reproductive needs. Males and females both use resources for flowering. After flowering, females continue to allocate resources to seed and fruit production, whereas males revert to vegetative growth. The added burden of fruit and seed production is thought to result in higher female mortality when resources are scarce. Observed mortality in drought years often does seem to fall more heavily on fruiting females than on males, in support of this idea.
An interesting twist to the tale is when marked vegetative differences occur between the sexes: males of some species typically have smaller leaves and more and finer branches than females. It has been suggested that these smaller leaves allow males to grow in drier places, leaving the choice sites for females - as all gentlemen should do! (No space herein for conflicting arguments!) Where they do grow together, males should not compete as intensely with females, so that sex ratios should be less biased than in species with identical sexes. Of course there are risks: if the sexes get too different they may end up being so far separated from their Entwives for aeons through such a distortion fo dioecy. Marloth reported a possible real-life example for Cliffortia ruscifolia (Rosaceae = Rose Family) on Table Mountain: all the individuals at lower elevations were males, and all at higher elevations female! (I suspect that this species is really monoecious with sequential flowering.)
William Bond, Botany Dept, UCT
Therefore, humans are `dioecious'. However, the majority of plants are neither dioecious nor monoecious, but
HERMAPHRODITIC (plants having flowers that contain functioning both male and female organs). Note that these terms refer to plants. Flowers are termed bisexual (two-sexes: having functioning both male and female parts) or unisexual (one-sex: having functional organs of only one sex). We will ignore the terminology for inflorescences (it is horrendous), and merely inform you that female plants of Aulax pallasia (and occasionally A. umbellata) have inflorescences which contain both functional female and sterile male flowers. For this reason John Rourke has suggested that the ancestors of Aulax must have been monoecious. Although the ancestors of Leucadendron must originally have been hermaphroditic, as in all other Proteaceae, we have no clues as to how it became dioecious.
Professor Roy Lubke (Grahamstown) has written to Phloeonomus to suggest how to find out more about "dioecy". Perhaps you would like to do this exercise, which I will definitely do - as soon as I feel energetic enough to leave my protea head.
Get a few of your friends together and go to an area which has some conebushes. Note that not all the conebush plants have cones: some plants (the mummies: females) have big cones in which they store their seeds on the plant, and other (the daddies: males) only produce flowers which they drop when they have finished flowering. Walk around and count the number of plants which are male and female. Record your results. Are there more males than females? Can you explain why? Are there more girls than boys in your class at school? What is the reason for these differences?
Read William's explanation to Roy's question? Then why not go and test William's theory. Count the male and female plants of a conebush species which has male bushes looking quite different from its female bushes. Males of these species are taller, with thinner, more branched stems, and smaller leaves. Compare this to a conebush species which has male bushes looking more like female bushes. Do your results support William's theory? Are there more females in some areas? Can you suggest another theory?
Want to play a cute little game with Stochasticity and Dioecy?
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