Botanist's Lens: X's and Y's of the Plant World

Monoecious Magnolia, simplest and one of the most primitive flower, Pistil (female) in the center higher than the stamen (male).

Flower to fruit, fruit to seeds, seed to seedling... the reproduction cycle in plants is simple enough and obvious. But what about the X's and Y's of the plant world? Nope, I am not talking about the Gen X and Gen Y.

Humans have 22 pairs of chromosomes and a pair of sex chromosomes: XX or XY. Are all flowers similar? In plants, although most of them have flowers with both male and female sex organs (stamen and pistil), there are many plant species with either only male or female flowers.

Monoecious Rose, pistil (female) in the center and stamen (male) in the outer ring.

Monoecious plants have male and female organs in the same flower, eg. Rose, Magnolias, and Hibiscus.

Monoecious Toad Lily, 'Tricyritis' flower
Female dioecious flower of Holly 'Ilex sps.'

Dioecious plants have unisex flowers, either male or female flowers. Common examples of dioecious plants include, Ilex, Ginkgo, Cannabis, Willows, etc. "Dioecy is a widespread condition in flowering plants, despite their recent evolutionary origin: 6% of the 240,000 angiosperm species are dioecious and 7% of 13,000 genera of angiosperms include dioecious species, suggesting that it has arisen many times during flowering plant evolution. Plants are key players in the study of the evolution of sex determination because they offer a unique opportunity in giving access to the very early stages of X and Y chromosome history. " Source

Male dioecious flower of Holly 'Ilex sps.'

Most dioecious plants are wind pollinated, since they don't have the male and female parts on the same flower. They don't depend on insects or birds for their pollen to transfer.

The chromosome constitution of male and female plants is 22 + XY and 22 + XX respectively. "The sex chromosomes in hepatics, mosses, and gymnosperms are morphologically heteromorphic. In angiosperms, heteromorphic sex chromosomes are found in at least 19 species from 4 families, while homomorphic sex chromosomes occur in 20 species from 13 families. The prevalence of the XY system found in 44 out of 48 species may reflect the predominance of the evolutionary pathway from gynodioecy towards dioecy. All dioecious species have the potential to evolve sex chromosomes, and reversions." Source Not all flowers are similar or are simple in any way. Since Dioecious flowers have unisex flowers, when adding these species to your garden, consider adding both females and males of that species. For example, Ilex verticillata, Winterberry has male and female flowers on separate plants. When planting Winterberry, look for compatible male and female species.

Complicated X's and Y's of the plant world? Yes! "Charles Darwin recognized that flowering plants have an unrivaled diversity of sexual systems. Determining the ecological and genetic factors that govern sexual diversification in plants is today a central problem in evolutionary biology." Source

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