A Close-up View of the Wildflower "Sweet Pea"


(Lathyrus latifolia)


by Brian Johnston   (Canada)

Over the last few years, I have systematically searched for wildflowers in the vicinity of my home.  Many species are ubiquitous.  What surprises me is the number that grow in only one location.  I often wonder what strange phenomenon led to their occurrence in a particular spot.  The sweet pea plants studied in this article grow in an area less than one square metre in the dappled shade of a stand of male staghorn sumac trees.  The plants bloom every year in early June, about a metre from the sidewalk shown in the image below. 



Unless you are looking carefully, the flowers may go unnoticed, since the nearby grass and weeds are taller, and surround them.  The blooms are two to three centimetres in diameter, and grow on straggly stalks to a height of no more than twenty five centimetres.





The peas belong to the family Fabaceae.  Many of the members of this family have become important commercial or ornamental crops, for example:  edible peas and beans, clover, and sweet peas. The sweet pea is native to Europe, but has been naturalized over most of North America.  Other common names for the species are the “everlasting pea” or the “perennial pea”.  The image that follows shows the two oval leaves joined by a short stalk to the main stem.  Two additional tiny leaflets are positioned at the joining point, (called the axil).



A group of immature sweet pea buds can be seen below.  Each bud displays a calyx composed of five pointed green sepals (modified leaves).  It is interesting to note that a tiny green filament-like thread grows out from the point of intersection of each bud’s stem to the main stem.



As the buds increase in size, they begin to show pink colouration.



Just before blooming, the buds display their natural bright red colour.



The large outer petal that will form the flower’s “banner” (see later), opens to reveal the inner petals of the flower.



Finally, the brilliantly coloured blooms are revealed. The upper-most petal has a noticeable vertical groove.  This petal forms the “banner” or “standard”of the flower.  Beneath the banner are two lateral petals referred to as “wings”.  It is these wings that cause the flower’s structure to be called papilionaceous, (like a butterfly).  Finally, beneath the wings are two petals joined to form the “keel”.  This keel encloses the reproductive structures of the flower.



A side view gives a different perspective .  Notice how the two wing petals effectively cover the top portion of the keel.



Although the flowers in my location are deep red dappled with white, the colour may range from red through pink to (rarely), white.



While photographing a sweet pea flower, I was struck by the elegant form of this remarkable bloom!



Photographed from the back, the flower shows a lighter colouration, and the five-pointed green calyx at the base of the petals is visible.



If the two petals forming the keel are gently removed by pulling on them, the reproductive structures hidden beneath are revealed.  Ten orange-brown anthers (male pollen producing organs), supported by white filaments, surround the single hairy green pistil formed by the stigma (female pollen accepting organ) supported by a sturdy style.  The flower structure of the sweet pea allows only one type of insect - the bee, to pollinate it.  The nectar in a flower is stored just over the filaments of the anthers.  As the bee enters the flower, its mass pulls down the keel, and the anthers and stigma move up to touch the underside of the bee’s body.  The bee gets the nectar, and in so doing, becomes coated with pollen.



Under the microscope, an anther can be seen to have many spherical to ellipsoidal pollen grains on its surface.



A photomicrograph of the tip of the pistil shows both the oval green stigma, and the hairy, lighter green style that supports it.  Many pollen cling to both stigma and style.



Pollen grains are irregular in shape.



The cellular structure of the banner petal shows both red and white cells in its makeup.



Pea flowered plants have the ability to obtain, (or “fix”) nitrogen from the atmosphere by using soil bacteria (Rhizobium).  It is this capability that makes such plants useful, both in the environment, and to farmers, by increasing soil fertility.


Photographic Equipment

About a third of the photographs in the article were taken with an eight megapixel Canon 20D DSLR and Canon EF 100 mm f 2.8 Macro lens.  An eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 6T, Sony VCL-M3358, and shorter focal length achromat used singly, or in combination), was used to take the rest of the macro images. The lenses screw into the 58 mm filter threads of the camera lens. Still higher magnifications were obtained by using a macro coupler (which has two male threads) to attach a reversed 50 mm focal length f 1.4 Olympus SLR lens to the F 828.  The photomicrographs were taken with a Leitz SM-Pol microscope (using dark-ground condenser), and the Coolpix 4500.


References

The following references have been found to be valuable in the identification of wildflowers, and they are also a good source of information about them.



 All comments to the author Brian Johnston are welcomed.


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