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Tuesday, September 28, 2010

Sparmannia africana, African Hemp

All plants are capable of gradual movement as a result of slow growth, but relatively few species can make the kind of rapid movements that we tend to associate with animals. The Venus fly trap Dionaea muscipula and the sensitive plant Mimosa pudica are the two species with rapid leaf movements that are most familiar but another fast-mover is African hemp Sparmannia africana. If you gently brush the cluster of golden stamens in the centre of the flower nothing happens for a second, than they move outwards, away from the stigma. The time delay between the photo above and that of the same flower whose stamens have been brushed, below, is about five seconds and you can easily see how the stamens have spread apart.
Presumably this is some kind of mechanism to aid pollination, although how it would do that isn't obvious. Similar stamen movements occur in Mahonia and Berberis flowers, although in those cases the stamens move rapidly inwards when they are stimulated.

Sparmannia africana comes from South Africa and makes a very fine house plant - if you have room for it. My plant grew too big for our conservatory and is now in Durham University Botanic Garden, where it has reached about three metres in height and is currently flowering and setting seed.

Friday, September 17, 2010

Mutants and Monstrosities

You don't need to be a gardener for very long before you discover that within every species there are always a few plants that don't conform to the norm. They're either mutants (where changes in their genes produce a different shape, form or colour of plant) or they're monstrosities (where there some external agency, like environmental stress or disease has led to abnormal growth).
Only mutants produce heritable changes, that will be passed on to at least some of their progeny. I would guess that the abnormal ox-eye daisy (Leucanthemum vulgare) above is a mutant. Some of its florets are spoon-shaped and since this abnormal plant turned up in my garden a few years ago others have appeared in subseqient years with the same characteristic, probably arising from seeds produced from those abnormal florets. Cultivars of a number of daisy-like species that have flowers composed entirely of spoon-shaped petals have been bred, so it's highly likely that this is a genetically controlled change in the pattern of petal development.

I'm pretty certain, on the other hand, that this two-headed ox-eye daisy is a monstrosity, the result of some agency that has interfered with the normal development of the flower and produced twin heads. Temperate-shock (i.e. frost) can sometimes do this.

This monstrous spear thistle Cirsium vulgare is the result of several flower heads becoming joined, side by side into a cock'scomb-like inflorescence, probably as a result of either stress during a critical phase of flower bud development or perhaps infection with a bacterium - Corynebacterium is known to cause a similar phenomenon, known as fasciation, in ...

Linaria purpurea...

Forsythia x intermedia, where it causes a switch in symmetry of the stem, from radial symmetry to flat, plank-like growth...

... and occasionally in herbaceous species like candelabra primulas ......

Some mutant forms of growth are widely cultivated for human consumption, like the cauliflower - which is a mutant inflorescence where the flower buds proliferate to the extent that they can no longer open properly.

My favourite monstrosity, though, is to be found in foxgloves Digitalis purpurea, where occasionally the last flower to form at the top of the floral axis results from fasciated growth of several flower buds. If you compare the monstrous flower on the left with the normal one on the right you can see that it has four stigmas, styles and ovaries, within a single tubular corolla - four flower buds fused to form a single flower.

Wednesday, September 8, 2010


Since this is the time of year when most plants in my part of the world are setting seeds, I thought I'd post a few images of these wonderful objects. This is one of the ripening seeds from my runner beans Phaseolus coccineus, with the testa and one of the cotyledons removed, revealing the embryonic plant inside. The cotyledons contain the store of proteins, carbohydrates and lipids that will provide sustanance for the germinating seedling until its first true leaves unfurl and begin to photosynthesise. In this image you can see that the rudiments of the venation in those first true leaves, that will transport sugars to other part of the young seedling, have already formed in the embryo. The embryo is attached to the cotyledons at its hypocotyl and the embryonic root points downwards and to the left. 

There's a heavy crop of oak (Quercus spp.) acorns in this part of Durham this year - it's a mast year. Oak acorns - technically nuts - germinate soon after they fall to the ground, anchoring themselves via their root but then suspending further growth until spring, when the shoot forms. If you are planning to plant oak trees, it's best to sow the fresh acorns soon after they fall.

Once seeds germinate their root needs to spear into the soil quickly, driven by the hydraulic force inside expanding cells. Here you can see the glistening root cap of a maize (corn) Zea mays seedling, producing the mucilage that lubricates its path between the abrasive soil particles. Further back, you can see the forest of root hairs, each a single elongated epidermal cell that makes intimate contact with the surface of soil particles and absorbs water and soluble minerals. Each root hair has a short life span, of perhaps a day, and new hairs are formed constantly behind the advancing root tip; collectively they constutive a vast absorptive surface.