Wednesday, 20 February 2013

Wednesday wildflower: stinky little cress

When I was a kid, a favourite activity was to plant out a mix of mustard (Brassica nigra) and cress (Lepidium sativum) seed on a wet flannel in a saucer.  In a few days, a lawn of seedlings could be harvested for delicious tangy sandwiches.  The first thing that happens is the seeds swell and the cress seeds develop a blob of jelly—mucilage— around them.  Many of the family (Brassicaceae) have specialised mucilage bodies in their epidermal cells, and the mucilage they produce both glues them to the substrate and surrounds them in a moisture-holding layer.
Wart cress, Lepidium didymum.
Cress is the best known of many species in the genus Lepidium.  We have a good range of both native species (more on them another day) and weedy introduced ones.  Until recently the genus has been characterised by having small squat seed pods that open to release one seed from each half.  The reduction to one seed in each half of the pod makes them quite different from most of the rest of the family, which have long skinny pods with many seeds in each half.  But a few other genera, notably Cardaria and Coronopus (wart cress), have one seed per half; the only difference is they don't open.  And lately these have been shown to be simply non-opening Lepidiums.  That is, they've evolved from Lepidium ancestors by losing the line of weakness that allows the fruits to open.  So now these small genera are included in Lepidium.
Lepidium didymum flowers have just two functional stamens, but they're just like many other Lepidium flowers.
We have two wart cresses in New Zealand, Lepidium didymum and L. squamatum.  In both, the fruit splits into two one-seeded portions, but the seeds are not released from the fruit.  They also smell pretty unpleasant; you wouldn't want to put wart cress in a sandwich.
Lepidium didymum fruits don't open; instead they split into two one-seeded portions.
Loss of dehiscence (opening) is actually pretty common in the plant world, and because sometimes it leads to a fruit that looks and behaves very differently from those of close relatives, it can lead to misleading classifications if those striking differences are emphasised.  These Lepidiums are closely related to different-looking plants that have opening fruits.
Cardamine lacustris fruits are short and squat and they open slowly, unlike the slender explosive ones of other species of Cardamine.
The same thing happened in Cardamine, another genus of Brassicaceae that's characterised by explosively-opening fruits.  In the days before DNA sequencing, I made the wrong call about a newly-discovered native plant.  It looked like a Cardamine, but its fruits opened without the characteristic explosion, and so I assumed that meant it wasn't a Cardamine.  I named a new genus, Iti, for this plant.  More recently, DNA sequencing has shown its true relationships and it's been reclassified in Cardamine, as C. lacustris (Heenan 2002).


Heenan, P.B. 2002. Cardamine lacustris, a new combination for Iti lacustris (Brassicaceae).  New Zealand Journal of Botany 40: 563–569

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