The Squirting Cucumber

Plants have gone to great lengths when it comes to seed dispersal. One of the most bizarre examples of this can be found in an ambling Mediterranean plant affectionately referred to as the squirting cucumber. As funny as this may sound, the name could not be more appropriate. 

Known scientifically as Ecballium elaterium, the squirting cucumber can be found growing along roadsides and other so-called "waste places" from the Mediterranean regions of western Europe and northern Africa all the way to parts of temperate Asia. It is the only member of its genus, which resides in the family Cucurbitaceae. It is a rather toxic species as well, with all parts of the plant producing a suite of chemicals called cucurbitacins. In total, it seems like a pretty unassuming plant. It goes through the motions of growing and flowering throughout the summer months but the real show begins once its odd fruits have ripened. 

A cursory inspection would not reveal anything readily different about its fruit. Following fertilization, they gradually swell into modest sized version of the sorts you expect from the gourd family. It's what is going on within the fruit that is most interesting. As the fruit reaches maturity, the tissues surrounding the seeds begin to break down. The breakdown of this material creates a lot of mucilaginous liquid, causing internal pressure to build. And I mean a lot of pressure. Measurements have revealed that at peak ripening, pressures within the fruit can reach upwards of 27 atm, which is 27 times the amount of atmospheric pressure we experience when standing at sea level!

A cross section of the fruit showing the weakened connection point.

A cross section of the fruit showing the weakened connection point.

At the same time, the attachment point of the stem or "peduncle" begins to weaken. With all that pressure building, it isn't long before something has to give. This is exactly the moment when the squirting cucumber earns its name. The stem breaks away from the fruit, revealing a small hole. Within a fraction of a second, all of that pressurized mucilage comes rocketing outward carrying the precious cargo of seeds with it. 

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The result is pretty remarkable. Seeds are launched anywhere from 6 to 20 feet (1 - 6 m) away from the parent plant. This form of dispersal falls under the category of ballistic seed dispersal and it is incredibly effective. Getting away from the competitive environment immediately surrounding your parents is the first step in the success of any plant. The squirting cucumber does just that. It is no wonder then that this is an incredibly successful plant species. 

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

The Explosive Dwarf Mistletoes

I used to think mistletoes were largely a southern phenomenon, preferring regions with mild or even no winters. Then I was introduced to the dwarf mistletoes in the genus Arceuthobium. These odd parasites can be found growing throughout the northern hemisphere. Their affinity for conifers has landed them on the watch list of many a forester yet, despite their economic implications, the dwarf mistletoes are fascinating parasitic plants. 

First and foremost, these are aggressive little plants. They vary in their host specificity. Some species can grow on a wide variety of conifer species from Abies balsamea (balsam fir), Larix laricina (American larch), to Pinus strobus (eastern white pine), whereas others are more specialized, preferring only spruces (Picea spp.). Regardless, infestations of these parasites can do some interesting things to conifer stands. 

Similar to other mistletoes, the dwarfs are stem parasites. They penetrate into their hosts vascular tissues and set up shop, sucking up water and photosynthates and giving nothing in return. Because of this, large infestations can seriously drain their host trees as they themselves have reduced or even no photosynthetic capacity. Additionally, they interfere with nutrient and hormone flows throughout the branches of their host. Such disruptions can result in the formation of dense clusters of branches called "witches brooms." Some dwarf mistletoe infestations can become so intense that they effectively girdle their host tree.

In natural settings, this serves an ecological function. By weakening their hosts, dwarf mistletoes can leave room for other plant species to take root. They also keep one species from becoming too dominant. As such, mistletoe infestations can actually increase plant diversity in the long run. Dwarf mistletoe infestations only become an issue once humans get involved. They can cause serious financial issues for foresters as well as damage important or valued specimen trees. In our highly fragmented forests, their natural behavior can get in the way of human ideals. 

All of this talk of damage can distract us from just how amazing some of these species really are from an organismal standpoint. For instance, the lodgepole pine dwarf mistletoe, Arceuthobium americanum, is capable of thermogenesis. Unlike the other examples of thermogenesis in the plant world, this has nothing to do with flowers. Instead, thermogenesis in A. americanum is used as a seed dispersal agent. 

The dwarf mistletoes don't rely on fleshy fruits to get their seeds from one tree to another. Instead, they utilize ballistic means. As their seed pods mature, they gradually swell. Once pressure is great enough, the seed pods erupt, sending their sticky seeds flying through the canopy at speeds of up to 62 mph (100 km/h)! If lucky, the seeds will stick to the branches of a viable host or be transported there in the fur or feathers of an animal. For A. americanum, the eruption of its seed pods is triggered by heat. Using specialized metabolic pathways at the cellular level, A. americanum is able to heat its seed pods up to ~2 °C warmer than its surroundings, thus triggering its pods to explode. 

Pretty incredible for a species so often labelled as a pest. 

Photo Credit: [1]

Further Reading: [1] [2] [3] [4]