Daily Sketches – June 30 Flowers – 13. Robinia pseudoacacia

Today I want to take a closer look at the Black Locust flower (Robinia pseudoacacia – the false acacia). Acacia grows only in sub-tropical and tropical areas and, although has leaves very similar to Robinia, the shape of the flower is different.

The flower of Robinia has a morphology typical of plants of the Fabaceae or Leguminosae family where we can find some very well known plants: Wisteria, Trifolium, Phaseolus (the bean), Lupinus, Vicia – that we just drawn – Medicago, Coronilla and so on. Therefore if you learn about the structure and shape of this flower, it will be very easy to draw the others.

1. Robinia pseudoacacia 2. Medicago sativa 3. Lupinus sp. 4. Coronilla (syn. Securigera) varia 5. Phaseolus vulgaris 6. Trifolium repens 7. Wisteria sinensis

The form is called Papilionaceous (from Latin: papilion, butterfly) and of course it refers to the butterfly-like shape of corolla. The flower have a bilateral symmetry (also called zygomorphic), that means it has only one plane of symmetry, only one line drawn through the middle produces two mirror-images halves.

Robinia pseudoacacia flower – bilateral symmetry

Corolla consist of five petals:

  • a single, large petal, standing upright, known as the banner (also flag, vexillum or standard petal)
  • 2 equal and smaller petals called wings (or alae)
  • and the carina, formed by 2 keel petals enclosing stamens and pistil

The stamens are found in two sets with a distinct shape: one of tube enveloping the pistil and has 9 filaments and anthers, then another with one single free filament, as you see in the picture. If you draw other plants from this family, do count the filaments, their number might vary.

All these flowers have developed specialized mechanisms for pollen release: only those insect which apply the right force on petals can release the stamens from inside the keel. An insect has to exert pressure on the flag petal in order to to be able to collect the nectar. While moving and pressing legs on the other petals to regulate body balance, it’s possible for the stamens and pistil to extrude, touching the the insect’s body.

One of the mysteries still being explored is how Apis mellifera manages to collect pollen and make that so popular acacia honey, given that it is not strong enough to trigger that stamens release mechanism. So, if you are a passionate naturalist and you have a black locust in the garden, that’s something to study… 

In today’s tutorial I’m drawing the butterfly-like form of the black locust flower.

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