Readers’ wildlife photos

Today we have another photo-and-text contribution from Athayde Tonhasca Júnior, this time about marijuana and its pollination.  Athayde’s text is indented, and you can enlarge the photos by clicking on them:

Blowing in the wind

The year 1936 is a landmark in the history of motion pictures: that’s when Reefer Madness was released to American theatres. [JAC: The whole movie is free on YouTube, here.]  The film tells the story of high school students experimenting with reefers (marijuana cigarettes), which spell their doom: innocent boys and girls are dragged into depravity, sexual abuse, organized crime, insanity, suicide and murder. Reefer Madness, intended as an educational feature, was voted by some critics as one of worst films ever made: unavoidably, it has become a cult classic.

The arrest of Ralph Wiley, a murderer who ended his life in an asylum for the criminally insane because of reefer madness © Motion Picture Ventures, Wikimedia Commons:

With time, attitudes towards marijuana changed drastically in some countries, from social acceptance to legalisation, removal from lists of most dangerous drugs, therapeutic applications and a lucrative industry. Revenue from medical, pharmaceutical and recreational cannabis products in the United States is forecast to reach over US$ 45 billion in 2025.

The cannabis plant (Marijuana sativa). ‘Cannabis’ and ‘marijuana’ are interchangeable; here, ‘cannabis’ will be used for the plant, and ‘marijuana’ for its drug product © US Fish and Wildlife Service, Wikimedia Commons:

Legalisation created other equally profitable opportunities: cannabis can be cultivated as hemp, which is a source of food products and a fibre used since ancient times for the manufacture of rope, paper, textiles and many other items. Despite coming from the same species, marijuana and hemp are widely different products. Crops for the marijuana or the hemp markets must contain respectively minimum and maximum levels of tetrahydrocannabinol (THC), the plant’s main psychoactive component – specific thresholds are determined by various local legislations. These legal limits have been a source of imbroglios among authorities and growers, all because cannabis is so good at being anemophilous, that is, wind pollinated.

Cannabis is dioecious – male and female flowers are produced on separate plants. But there’s a quirk in cannabis’ sex life. If a mature female flower is not pollinated, its style and stigma (the pollen-receptive parts) grow and expand in length and girth. The longer the virginity period, the more flowers a plant produces and the larger they become. From an evolutionary perspective, this expansion of female parts could be a strategy for increasing the probability of fertilisation when male plants are scarce (Small & Naraine, 2016). Floral mass has an enormous practical implication: female flowers contain the highest concentrations of THC and other cannabinoids, terpenoids and flavonoids, the secondary metabolites responsible for cannabis’ pharmacological and psychoactive properties.

Cannabis female flowers © Erik Fenderson, CABI Digital Library:

Male plants are not chemically endowed as females, but they present a much bigger problem than having low concentrations of cannabinoids. Being male, they can of course pollinate, and by doing so they can wreck a whole cannabis crop. Fertilised female plants produce smaller flowers with significantly lower levels of secondary metabolites. Since the time cannabis cultivation was solely in the hands of outlaws, growers learned to remove male plants to assure yields and drug potency. Today’s growers may also plant feminised seeds (bred to contain no male chromosomes) or female clones, all to keep a male-free environment. But these efforts may come to nought if a neighbouring farmer decides to go into the hemp business.

Harvesting hemp fibre © John McPartland, CABI Digital Library:

A hemp grower has no reason to weed out male plants; they are equal to females regarding fibre production, and it would be too costly to do so. Unfortunately for marijuana farmers, pollen grains from a hemp crop are likely to find their way to their female virgin plants.

A cannabis pollen grain is about 30 µ (1 μ = 1/1,000 mm) in size, and there are lots of them. A large male plant can produce hundreds of flowers, each one releasing up to 350,000 pollen grains into the wind. A 0.4 ha hemp plot can cover the ground up to 400 m away with 1.7 grains/cm² per day, or 357,000 grains/m² over three weeks (Small & Antle, 2003). That means that any female cannabis plant within reach is likely to be fertilised. And “within reach” means a lot of ground, in the range of dozens of kilometres or more for recorded cases. Cannabis pollen from Morocco made it all the way to Spain, over 200 km away (Cabezudo et al., 1997). Even before marijuana and hemp were legalised in America, their pollen, likely originated from wild hemp or illegal crops, made up 36% of all airborne pollen counts in Midwest states (Stokes et al., 2000).

Scanning electron microscopy of fresh (A,B) and dried (C,D) pollen grains of a cannabis male flower © Olejar & Park, 2022:

The contamination of female cannabis plants by dispersing pollen has caused significant financial losses, lawsuits and crop destruction court orders in America (Nimmala et al., 2024). Some state and local authorities are stepping into the melee by banning hemp or marijuana farms, and some farmers are growing their crops indoors with air filtration systems, which increase costs sharply.

The dispersal capability of cannabis pollen is a source of frustration and anxiety to American farmers. But it highlights the efficacy of anemophily, even though it doesn’t at first seem to be the case: this strategy consists of broadcasting huge numbers of pollen grains for the odd chance of some of them finding receptive flowers, events that depend on wind, humidity, rain and temperature. And yet, about 10% of all angiosperms (flowering plants) from a range of families share this apparently wasteful mode of reproduction, which derived from entomophily (insect pollination). That such evolutionary transition happened for so many plants puzzled Darwin: “It seems at first sight a still more surprising fact that plants, after having been once rendered entomophilous, should ever again have become anemophilous“(Darwin, 1876). Darwin himself had an inkling of the reason for that: “If any entomophilous species ceased altogether to be visited by insects, it would probably perish unless it were rendered anemophilous, or acquired a full capacity for self-fertilisation.” Indeed, shortcomings such as absence or scarcity of pollinators, or inadequate pollen deposition, are believed to be the evolutionary driving force towards anemophily (Friedman & Barrett, 2009).

Wind-pollinated plants. Left: common ragweed (Ambrosia artemisiifolia) male and female groups of flowers. Centre: staminate (male) flowers of common walnut (Juglans regia). Right: pistillate (female) flowers of common walnut showing the exposed stigmas © Digital Encyclopedia of Ancient Life:

We may think that anemophilous plants are not relevant to insects, but that’s not quite right.  Several species, including important crops, are visited by bees and hover flies for pollen or nectar (Saunders, 2017). As these plants tend to flower earlier than insect-pollinated ones, they are important sources of food during the spring hunger gap, when there’s a dearth of flowering plants.

An early mining bee (Andrena haemorrhoa). As its common name suggests, this is a spring-flying bee that takes pollen from early season anemophilous plants © Charles J. Sharp, Wikimedia Commons:

Some wind-pollinated plants take advantage of insect visitors by relying on them to do some of the pollen transfer. These are ambophilous species, meaning they are pollinated by wind and insects. This mode of reproduction could be a stable strategy or an intermediate state from entomophily to anemophily: we don’t know for sure. What we do know is that ambophily is more common than previously thought (Culley et al., 2002). At least 120 angiosperm species – the true number is likely to be much higher – are ambophilous, including plants long thought to be anemophilous and important crop species such as coffee (Coffea spp.), avocado (Persea americana) and oil palm (Elaeis guineensis) (Abrahamczyk et al., 2023).

Common heather (Calluna vulgaris), the dominant plant in most European heathland and moorland, is mainly pollinated by bumble bees, honey bees, hover flies and the wind © Aqwis, Wikimedia Commons:

The difficulty cannabis farmers in America face in keeping pollen out of their crops illustrates the efficacy of anemophily. Traditionally, insects have been considered outsiders from this mechanism of plant reproduction. We know now that plants and insects can benefit themselves and each other from blowing winds.