Vernal Equinox

Yesterday was the vernal equinox so today is the first day of 2019 that is longer than the previous night. The days will get longer and the nights shorter until the summer solstice: June 21st or thereabouts. For the park this is a time of extraordinary growth.

It used to be believed that day-length was a trigger for many flowering plants, a timing mechanism for a species so that all the individual plants in a population synchronised their flowering and therefore maximised the opportunities for successful cross-fertilisation. A neat trick. The most recent research, however, has shown that it is the length of the dark periods that actually control plant growth.

The relative lengths of daylight and darkness are called photo periods and a plants ability to respond to them is called photoperiodism. There are pigments in the plant’s leaves, called phytochromes, which enable the plant to measure light and darkness.

For a flowering plant, the timing of the transition from vegetative growth to reproduction is critical. Not only must it synchronise with other plants of the same species to ensure pollination but also with sometimes very specific pollinators. Seed development, dispersal and germination can also need careful timing.

Photoperiods are important seasonal clues for both our flora and fauna; we are only just beginning to understand they work.

Header photograph NASA

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