(Clammy refers to the stickiness of the plants.)
Spider-flower, aka Cleome (Cleome hassleriana and relatives), is a garden flower like a friendly ghost haunting my memories of gardens long ago and far away. That lovely cultivated species has a locally native mini-me, Clammyweed (Polanisia tenuifolia). The thundering arrival of the rainy season has given Clammyweed a boost, making it a joy to encounter all spunky in scrub habitats.
It has a floral oddity. On any individual the flowers tend to be a mix of bisexual and male-only. Put differently, all flowers have pollen-producing stamens (“male”), but only some have both stamens and “female” pistils (seed-making organs). The several stamens are easy to recognize, being bright yellow. The pistils look like bent fingers curving upward across the face of the flower to become the long skinny pods characteristic of Polanisia, and of Cleome.
The term for a mix of bisexual and male-only flowers on the same plant is andromonoecy (ANN-dro-MON-ee-cee), a word you’ll forget before the next paragraph. But before you forget it, what good is andromonoecy? Why would a species combine bisexual flowers with male-only flowers on the same individuals? It’s rare.
Over the decades, various botanists have floated possible explanations. The prevailing thought, laced with some speculation, is that because female flowers are “expensive,” a plant can sustain only a limited number of them, especially in a tough habitat like scrub. That is, fruits and seeds resulting from a female flower make big demands a parent plant’s limited resources, like too many children in a human family. In the thirsty nutrient-deprived scrub a stressed plant can afford only so-many plump viable seeds. And it gets worse: each fruit contains lots of seeds, with each seed requiring a successful pollen grain delivery. You need a good bit of pollen even if fruiting is limited.
So then let’s see, redundantly. Because the scrub-stressed plant can sustain only “so many” fruits filled with nutrient-demanding seeds, it is counterproductive to make too many bisexual flowers. But still the plant needs plenty of pollen for even the constrained seed crop, augmenting the pollen supply by making male-only flowers. Moreover, male-only flowers offer the added benefit of adding to the plant’s scent and visual display, and offer nectar and pollen food rewards. “Hey bugs, get some nectar and pollen, and while you’re here drop off some pollen on that bisexual flower.”
That invites the question of, is pollen from male-only flowers able or likely to fertilizer bisexual flowers on the same plant? Neither answer is a “deal breaker,” and data from similar situations suggests same-fertilization to be possible but unusual, an experiment waiting to be tried on Polanisia tenuifolia.