Himalayan snowball plants and their fashionably functional coats

Credit to CGTN Nature film crew

Hairy plants are both fun and functional. Hairs or trichomes on the leaves of plants can serve a variety of functions. If the plant is growing in a region prone to cold temperatures, it is thought that a dense layer of hairs can function like a wool coat, keeping the plant warm when temperatures drop. This is such a popular idea that it is often assumed rather than tested. For a strange group commonly referred to as Himalayan snowball plants, the truth is a bit more complicated.

Himalayan snowball plants are members of the genus Saussurea, which hails from the family Asteraceae. Though the genus is widespread, the Himalayan snowball plants are confined to high elevation, alpine habitats in central Asia. As you can imagine, life at such altitudes is defined by extremes. Temperatures during the day can skyrocket due to the lack of atmospheric insulation. Conversely, temperatures can take a dive as weather changes and/or the sun goes down. One look at the Himalayan snowball plants tells you that these plants are wonderfully adapted to such habitats. But what kind of advantages does that this coat of hair provide?

Credit to CGTN Nature film crew

Well, research has revealed a bit more nuance to the whole “winter coat” idea. Indeed, it does appear that the furry coat does in fact provide some insulation to the plant. However, most of the warmth appears to come from the dark color of the inflorescence rather than by pure insulation alone. After all, the vast majority of plants do not produce any heat. The flower heads or capitula of these daisy relatives is low in stature. This keeps it out of the way of the coldest winds. Also, they are so deeply violet in color that they can appear black. This is no accident. As anyone can tell you, darker colors absorb more heat and that is exactly what happens with the Himalayan snowball plants.

Another interesting thing to consider is that most of the growth and reproduction in these plants occurs during frost-free periods of the year. Though temperature swings are frequent, it rarely gets cold enough to severely damage plant tissues until long after the plants have flowered and set seed. Moreover, there is some evidence to suggest that the dense coat of hairs may have a cooling effect during periods of intense exposure to sunlight. Their light color may reflect a lot of the incoming radiation, sparing the plant from overheating. Therefore, it appears that the benefit of such a thick coat of hairs has more to do with avoiding temperature swings than it does ensuring constant warmth. By buffering the plant against huge swings in ambient temperature, the hairs are able to maintain more favorable conditions for plant growth and reproduction.

Credit to CGTN Nature film crew

Also, because this area experiences a monsoon season during growth and flowering of Himalayan snowball plants, these hairs may also serve to repel water, keeping the plants from becoming completely saturated. If water were to stick around for too long, it could open the plant up to pathogens like fungi and bacteria. It could also be that by insulating the plant against temperature swings, the hairs also provide a more favorable microclimate for pollinators. Bumblebees are thought to be the main pollinators of Himalayan snowball plants and despite their ability to maintain higher internal temperatures relative to their surroundings, anything that can buffer them as they feed would be beneficial to both the bees and whatever plant they may be pollinating as a result.

Photo Credit: [1]

Further Reading: [1] [2]

Your string of pearls (and its cousins) are all members of the daisy family

Photo by LynnK827 licensed by CC BY-NC-ND 2.0

I love the spike in popularity of houseplants. The more popular indoor gardening becomes, the more plants become available for obsessive growers such as myself. If you are like me, then learning about the ecological and evolutionary history of the plants you keep makes them all the more special. Take, for instance, a small group of scrambling succulents affectionately referred to as “string of pearls,” “string of bananas,” and “string of tears.” These all make incredible houseplants if given the proper care, but they become all the more interesting when you realize that they are distant cousins of the dandelions growing in your yard.

That’s right, each of these species are highly derived members of the daisy family (Asteraceae). Their taxonomy has been a bit wonky over the years. When I first took interest in these succulents, they resided in the genus Senecio. Some authors have suggested moving them into the genus Kleinia or Cacalia, but current systematics suggests they belong in a genus of their own - Curio. Inspection of the relationships within this group reveals that closely related species have evolved slightly different growing habits. The plants I will be focusing on for this article each resemble creeping vines but many of their close relatives are less vine-like but nonetheless still creep along the ground. For the sake of this piece, I am going to stick with the genus Senecio because, regardless of their taxonomic placement, the “sting of” clade is super fascinating from an ecological standpoint.

Senecio citriformis photo by Salchuiwt licensed by CC BY-SA 2.0

Senecio radicans photo by KENPEI licensed by CC BY-SA 3.0

All of these stringy plants hail from arid regions of South Africa. In the wild, they mostly scramble over rocks and bushes, often emerging out of cracks in rock in search of the right microclimate. Their oddly shaped, succulent leaves are an evolutionary adaptation to the tough conditions in which they evolved. The most leaf-like anatomy belongs to that of the string of bananas (S. radicans). Each leaf of S. radicans is shaped like a tiny green banana. More extreme versions of leaf morphology are found in the string of tears (S. citriformis) and string of pearls (S. rowleyanus & S. herreianus). The leaves of these three species resemble peas in shape, size and color. The leaves of S. rowleyanus are more spherical in shape (pearls), whereas the leaves of S. citriformis taper towards the tip (tears).

Senecio herreianus photo by Frank Vincentz licensed by CC BY-SA 3.0

Though all of these species grow in dry habitats, the more spherical shaped leaves of S. rowleyanus and S. citriformis are thought to be best adapted for drought. In growing spherical leaves, these plants are taking advantage of the surface area to volume ratio of a sphere. The benefit of this is that these species are able to maximize water storage while minimizing the amount of leaf surface exposed to the blistering sun. This way the leaves are able to maintain high levels of photosynthesis without overheating, all the while reducing leaf temperature.

In each of these species, the surface or adaxial side of the leaf exhibits a translucent window that runs the length of the leaf. It has long been hypothesized that leaf windows allow light to transmit into deep into the interior of the leaf where the photosynthetic machinery resides. More recent experiments on window-leaved succulents suggests that reality is not that simple. Instead, these windowed surfaces appear to allow the plant to maintain healthy levels of photosynthesis without the damaging their leaves via overheating.

Photo by Frank Vincentz licensed by CC BY-SA 3.0

When plants reach maturity, flowering can be prolific. Thin stems topped with tiny composite heads of cream-colored flowers erupt from the mat of vegetation. Then and only then do these plants readily reveal their placement within the daisy family. The inflorescence is made up entirely of discoid flowers. There are no rays like that of a sunflower. The flowers themselves are said to produce a pleasant odor frequently described as sweet and spicy. After pollination, the flowers give way to seeds topped with a parachute-like pappus that will carry them far and wide on the wind.

Learning about the natural history of these plants has given me a whole new appreciation of these strange, succulent members of the daisy family. What’s more, there is a whole world of succulent asters out there (a post for a later time) and many of them are equally as fascinating and beautiful.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

An Intriguing Way of Presenting One's Pollen

Photo by Monteregina (Nicole) licensed by CC BY-NC-SA 2.0

Getting pollen from one flower to another is the main reason why flowers exist in the first place. It makes sense then why pollen is often made readily available to pollinators. For many flowering plants, this means directing the pollen-filled anthers outward where they are ready to take advantage of floral visitors. The sunflower family (Asteraceae) does this a bit differently than most. They utilize a technique called secondary pollen presentation.

Though secondary pollen presentation is not unique to the sunflower family, their abundance on the landscape makes it super easy to observe. For the sunflower family, what looks like a single flower is actually an inflorescence made up of dense clusters of individual flowers. Each individual flower is roughly tubular in shape and, oddly enough, the anthers are tucked inside the tube facing the interior of the flower. It may seem odd to hide the anthers and their pollen inside of a tube until you see the blooming process sped up.

Photo by László Németh licensed by CC BY-SA 3.0

The sunflower family actually relies on the female parts of the flower to bring the pollen out from the floral tube and into the environment where pollinators can access it. Members of the sunflower family are protandrous, meaning the male parts mature before the female parts. What this means is that the style of the flower can be involved in presenting pollen before it becomes receptive to pollen. This allows enough time for pollen presentation and reduces the likelihood of self pollination.

As the style elongates within the floral tube, one of two things can happen with the pollen inside. In some cases, the style acts like a tiny piston, literally pushing the pollen out into the world. In other cases, the style is covered in tiny, brush-like hairs that rake the pollen from the sides of the floral tube and carry it out as it emerges. In both cases, the style remains closed until enough time has passed for pollen to be taken away from the inflorescence.

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After a period of time (which varies from species to species), the style splits at the tip and each side curls back on itself to reveal the stigmatic surface. Only at this point in time is are the female parts of the flower mature and ready to receive pollen. With any luck, much of the flowers own pollen would have been collected and taken away to other plants.

The combination of sequential blooming of individual flowers and protandry mean that members of the sunflower family both maximize their chances of pollination and reduce the likelihood of inbreeding. Add to that their ability to disperse their seeds great distances and myriad defense strategies and it should come as no surprise that this family is so darn successful. Get outside and try to witness secondary pollen presentation for yourself. Armed with a hand lens, you will unlock a world of evolutionary wonders!

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

Further Reading: [1] [2]

The Dual Benefits of Smelling Like Frightened Aphids

Photo by KENPEI licensed under the GNU Free Documentation License

If you garden, you have probably dealt with aphids. These tiny sap-suckers not only drain the plant of valuable sap, they can also serve as vectors for disease. Plants must contend with the ever-present threat of aphid infestation throughout the growing season and have evolved some amazing defenses against these insects. Recently an incredible form of defense against aphids has been described in pyrethrum (Tanacetum cinerariifolium) and it involves smelling like a frightened aphid colony.

Aphids produce their own alarm pheromones when attacked. Because aphids form large, clonal colonies, these pheromones can help warn their kin of impending doom. Other aphids will also eavesdrop on these alarm signals and will avoid settling in on plants where aphids are being attacked. Aphids aren’t the only ones honing in on these scents either. Aphid predators and parasitoids will also use these compounds to locate aphid colonies. As such, these pheromones are helpful to the host plant because it can mean a reduction in aphid numbers.

An alate (winged) green peach aphid (Myzus persicae).

The selection pressured imposed by aphids on plants is so strong that it appears that at least one species of pyrethrum has actually evolved a means of producing these pheromones themselves. Pyrethrum is a member of the aster family (Asteraceae) native to southern portions of Eurasia. Like all flowering plants, its flowers are the most precious organs. They are the key to getting their genes into the next generation and therefore protecting them from herbivore damage is of utmost importance.

It has been discovered that pyrethrums produce an aphid alarm pheromone called ( E )-β-farnesene or EβF for short. The pheromone is not produced in every tissue of the plant but rather it is concentrated near the inflorescence. What’s more, pheromone production is not constant throughout the duration of flowering. Researchers found that production reaches its peak just before the inflorescence opens to reveal the flowers within.

Photo by そらみみ licensed under CC BY-SA 4.0

The production of EβF in pyrethrum appears to serve a dual function. For starters, it actually results in reduced aphid infestation during the early stages of flowering. When the initial aphid attack begins, these insects consume some of the EβF as they feed and release it as they excrete honeydew. Other aphids detect EβF within the honeydew and will actually avoid the plant, likely due to the perception that the aphids feeding there are already under attack.

That does not mean that predators are not to be found. In fact, the other benefit of producing EβF in the inflorescence is that it appears to lure in one of the most voracious aphid predators on the planet - ladybird beetles. The ladybird beetles are able to detect EβF in the air and will come from far and wide to investigate in hopes of finding a tasty aphid meal. The ladybird beetles were most frequently found on plants during the early stages of floral development, which suggests that EβF production in the floral tissues is the main attractant.

A 7-spot ladybird beetle (Coccinella septempunctata). Photo by S. Rae licensed under CC BY 2.0

Interestingly, it has been found that constant production of EβF is less effective at deterring aphids than pulses of EβF. It is thought that just as humans can get used to certain background levels of scent, so too can aphids. If aphids are exposed to high levels of EβF for long periods of time, they simply recognize it as the safe background level and will continue to feed. This may explain why pyrethrum plants only produce EβF for a short period of time during the most crucial stages of floral development. Research like this not only improves our understanding of the myriad ways in which plants defend themselves, it also offers us new avenues for researching more natural ways of defending the plants we rely on from unwanted pests.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]

Everlasting or Seven Years Little

Photo by Andrew massyn licensed under CC BY-SA 3.0

Common names are a funny thing. Depending on the region, the use, and the culture, one plant can take on many names. In other situations, many different plants can take on a single name. Though it isn't always obvious to those unfamiliar with them, the use of scientific names alleviates these issues by standardizing the naming of things so that anyone, regardless of where they are, knows what they are referring to. That being said, sometimes common names can be entertaining.

Take for instance, plants in the genus Syncarpha. These stunning members of the family Asteraceae are endemic to the fynbos region of the Eastern and Western Cape of South Africa. In appearance they are impossible to miss. In growth habit they have been described as "woody shrublets," forming dense clusters of woody stems covered in a coat of woolly hairs. Sitting atop their meter-high stems are the flower heads.

Each flower head consists of rings of colorful paper-like bracts surrounding a dense cluster of disk flowers. The flowering period of the various species can last for weeks and spans from October, well into January. Numerous beetles can be observed visiting the flowers and often times mating as they feed on pollen. Some of the beetles can be hard to spot as they camouflage quite well atop the central disk. Some authors feel that such beetles are the main pollinators for many species within this genus.

Photo by JonRichfield licensed under CC BY-SA 3.0

Their mesmerizing floral displays are where their English common name of "everlasting" comes from. Due to the fact that they maintain their shape and color for a long time after being cut and dried, various Syncarpha species have been used quite a bit in the cut flower industry. A name that suggests everlasting longevity stands in stark contrast to their other common name. 

These plants are referred to as "sewejaartjie" in Afrikaans, which roughly translates to "seven years little." Why would these plants be referred to as everlasting by some and relatively ephemeral by others? It turns out, sewejaartjie is a name that has more to do with their ecology than it does their use in the floral industry.

As a whole, the 29 described species of Syncarpha are considered fire ephemerals. The fynbos is known for its fire regime and the plants that call this region home have evolved in response to this fact. Syncarpha are no exception. They flower regularly and produce copious amounts of seed but rarely live for more than 7 years after germination. Also, they do not compete well with any vegetation that is capable of shading them out.

Photo by Andrew massyn licensed under CC BY-SA 3.0

Instead, Syncarpha invest heavily in seed banking. Seeds can lie dormant in the soil for many years until fires clear the landscape of competing vegetation and release valuable nutrients into the soil. Only then will the seeds germinate. As such, the mature plants don't bother trying to survive intense ground fires. They burn up along with their neighbors, leaving plenty of seed to usher in the next generation.

Research has shown that its not the heat so much as the smoke that breaks seed dormancy in these plants. In fact, numerous experiments using liquid smoke have demonstrated that the seeds are likely triggered by some bio-active chemical within the smoke itself.

So, there you have it. Roughly 29 plants with two common names, each referring back to an interesting aspect of the biology of these plants. Despite their familiarity and relative ease of committing to memory, the common names of various species only get us so far. That's not to say we should abolish the use of common names altogether. Learning about any plant should be an all encompassing endeavor provided you know which plant you are referring to.

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

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

Pearly Everlasting

Photo by Pendragon39 licensed under Public Domain

I have gardened with a lot of native plants over the years but pearly everlasting (Anaphalis margaritacea) may be one of my favorites. Not only is it easy to grow, this tough little plant can handle some pretty harsh soil conditions. In the wild, I often find it growing along gravelly roadsides where it puts on quite a show. Let's be honest with each other, who doesn't love a fuzzy plant.

Pearly everlasting is a member of the largest dicot family on the planet, the asters. As such, what appears to be single flowers doing their best imitation of a sunny side up egg is actually a collection of many tiny flowers clustered together to look like one big one. In a sense, this is a form of floral mimicry.

What is most unique about pearly everlasting is that it is dioecious. Individual plants produce disks that are either male or female. I can't really think of other asters that adopt this strategy. And what an awesome strategy it is. Being dioecious means cross-pollination. The reproductive disk flowers are those yellow ones in the center. The pearly white outer ring of each inflorescence is actually made up of a dense cluster of involucre.

© 2009 Walter Siegmund licensed under GNU Free Documentation License, Version 1.2

Did I mention this plant is fuzzy? Dense trichomes cover the stem and underside of each leaf. Hairs like this are adaptations to reduce water loss and overheating. However, there is evidence that in pearly everlasting, these hairs can also reduce feeding by spittlebugs. Nymphs looking for a tasty plant to drill into cannot seem to penetrate the dense growth of trichomes, which means each pearly everlasting gets to hold on to its sap.

Again, I can't speak highly enough about this species. It is native to much of North America and, in this writers opinion, should be in the drier portions of every native garden. All you need are a handful of seeds and a small population of pearly everlasting will soon be keeping you company.

Photo Credit: Wikimedia Commons

Further Reading: [1] [2]