Monday, March 7, 2022

Monoecious Monday: Trypsacum dactyloides

I like taking macrophotographs of grass reproductive structures, and when I was taking some pics of the native ornamental Trypsacum dactyloides (Eastern Gamagrass) I noticed something different. 

Grass flowers are notoriously tiny, and in fact the flowerheads that we see at the macro scale are aggregations of so-called "spikelets", which may contain one to many "florets" (the actual flowers). 

Male (anther) and female (stigma) structures in Imperata cylindrica (cogon grass)

All the florets that I had seen so far have been perfect, with each floret having both male and female parts. 

The male part is the stamen, which is composed of a filament holding up an anther, where the pollen resides. The female part is the pistil, which contains an ovary, a style, and a stigma. The anther and the feathery stigma are what people usually see easily when doing macro of grass flowers.

Feathery stigma and anthers hanging from filaments in Phalaris arundinacea (Reed Canary Grass)

The spikelets of T. dactyloides were surprising to me because at first glance I noticed that the male and female parts seem to be located on different parts, and thus different flowers!

Trypsacum dactyloides is a native grass here in Florida that is widely used as an ornamental, both in city street plantings, and in suburban areas. It is an attractive plant, with wide dark green leaves and a kinda roundish habit.

It turns out that this species, unlike many other grasses, is monoecious, with imperfect (or unisexual) flowers. Monoecious means that there are both male and female flowers on the same individual plant, and unisexual flowers have only either male or female parts. 

Stigma of T. dactyloides

In T. dactyloides, the male florets are situated on the upper part of the grass inflorescence, while the female flowers are located in the lower part of the same inflorescence. This is in contrast to bisexual flowers, which contain both male and female reproductive organs, and which I am more familiar with from my previous photos of other grasses.

Purplish anthers of T. dactyloides
Another grass species which might be more familiar to people and which also is monoecious is maize (Zea mays). In this grass, the male flowers are seen as long "tassels" on the top of the plant, while the female flowers are lower down in the plant and are the ones that form the "ear". The latter is what develops into the corn cob that we all like to eat.

And just to add to the confusion, it should be noted that there are dioecious grasses, although this is even rarer. In this case, the separate male and female flowers are also on separate individual plants! An example of a dioecious grass is Bouteloua dactyloides (buffalo grass). Due to the male and female structures being on different plants, this species needs to cross-pollinate.

But whatever the type, all these species are successful in what they do, given that they are all thriving in today's competitive natural world.

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