A Fruit and Nut Case

All flowering plants have seeds in some form or another but there is a huge diversity in size, shape and structure.  This can range from the almost microscopic seeds of orchids to the famously gigantic nuts of the coco de mer palm, native to only two islands in the Seychelles, which can weigh up to 30kg.  This diversity is further enhanced by the variation in the amount of material added around the seeds by the parent plant to form a ‘fruit’ as well as the seeds being clustered together in pods and fruiting bodies.  With the exception of plants like orchids that rely on associations with mycorrhizal fungi to support germination, the seed contains the dormant embryo and enough nutrients to support germination of the seed and formation of the initial root and first leaf-like structures (cotyledons) before the true leaves start to develop.

Blowing in the wind

In general, plants that are annual or short-lived like grasses, produce large numbers of small seeds to try and increase the likelihood of the seed ending up in a favourable place for germination.  Small seeds are also more likely to be scattered by the wind and many plants like dandelions and thistles have evolved features on their seeds that are specifically shaped to enhance the ability of the wind to distribute them.  Sometimes, with suitable weather conditions, this can spread the seeds over many miles and enable them to rapidly colonize new areas of suitable habitat.  Its not just the small herbaceous plants that take advantage of the wind to distribute their seeds and many will be familiar with the mass of fluffy material that is released by willows and poplar trees at the start of summer.  Birches also produce small seeds that are easily blown around in the wind and are exceptionally good at colonizing areas of bare ground and even loose mortar joints in walls and chimneys.

A ticket to ride

Another means that plants use to enhance seed distribution is to hitch a ride on a passing animal.  This occurs, mainly accidentally, with grass seeds but some plants have evolved burred seeds with hooked spines that are specifically designed to catch on animal hairs.  Most of us are familiar with picking up cleavers on our clothes from walking along overgrown footpaths in late summer but the seeds of plants like Forget-me-not, Enchanters Nightshade, Agrimony and Herb Bennet are also good at attaching themselves to your clothes after a country walk.  Perhaps the most tenacious of the burs are the seeds of Greater Burdock which can get so tangled in an animal's hair that they can be rather difficult to remove

Winged assistance

Larger seeds can contain more nutrients and therefore sustain growth for longer which means that there is more time for their roots to become established and consequently a greater chance of successful germination.  By developing special winged seeds some trees produce larger seeds that can still take advantage of wind distribution.  Ash, Hornbeam and Sycamore, as well as most conifers, are good examples of trees that successfully exploit this strategy. 

Free fall

Very large seeds are often referred to as nuts because, to protect the seed, the seed coat has become modified into a hard casing to protect them from being eaten before they can get to a suitable place to germinate.  Hazel nuts and walnuts are probably the best-known but the seeds inside the fruits of damson plums and hawthorns, also have very hard shells.  Not all large seeds have protective shells and some, like chestnuts and Beech, rely on the parental tissues to protect the seed until it is ripe.

The numbers game

Another tactic plants employ is, in some years, to produce extremely large numbers of seeds, overwhelming the ability of feeding animals to eat them all and thereby ensuring that some can successfully germinate.  Beeches are famous for having these ‘mast’ years because of the huge numbers of beech seeds (Beech masts) that can be produced.  Locally, this year was a moderate Beech mast year and I expect that next spring, we will see a lot of Beech seedlings growing amongst the leaf litter in our woodlands.  Many people have noticed that last year was an exceptional acorn year and in some places the ground under oak trees was completely covered in acorns making some paths feel like you were walking on a layer of marbles.   Interestingly, this year, I couldn’t find any acorns on common oaks (sessile or pedunculate) and could only find one or two on some young, non-native, Holm Oaks (Quercus ilex) that we have in our garden.  I hypothesize that the lack of acorns is a consequence of the weather conditions at flowering time in combination with ‘exhaustion’ from producing all those acorns last year.

Unpalatable seeds

Unsurprisingly plants also utilize their special ‘chemical warfare’ tactics to protect their seeds and discourage animals from eating them.   For example, legume seeds contain special carbohydrate binding proteins (lectins) which can cause problems for animals when digesting them and acorns contain phenolic compounds, known as tannins, which bind with proteins and some carbohydrates to prevent them being digested.  Despite this, some animals’ digestive systems have adapted to tolerate these chemicals and defeat the plant’s defences.

Fruitful associations

For plants, a potential downside of larger seeds is that it takes more of the plants energy to produce them and also, because of the increased amounts of stored nutrients, they become much more attractive as a food source for animals.  It is this balance of germination success and the risk of being eaten which has created a fascinating and complex relationship between plants and animals where the plant provides nutrition for animal and the animals accidentally help with seed distribution.  This is the primary reason why plants produce fruits, a nutritious covering of the seed created by the parent plant and specifically designed to attract animals to eat them.   When the seed is eaten with the fruit and passed intact through the digestive system it will end up some distance away, much further than it would be able to travel if it just fell from the plant.  To be successful, this approach can only work if the animal is large enough to swallow the seed whole and has a digestive system that does not damage it.  Fruits of blackberries are very well distributed by this process as the small hard seeds pass through most animals unharmed and, with luck, end up in a nice pile of poop which will provide lots of nutrients for the germinating seeds.  Even seeds as big as those inside damsons and sloes can pass through the digestive systems of large mammals like foxes and badgers.  Birds are often a major target for this process and fruits are often brightly coloured, usually red, to attract certain types of bird.  Some of these fruits or berries also have chemicals that are unpalatable to certain species, further refining the specific animals that are targeted; with the added potential side benefit that the non-target species will probably regurgitate the seeds quickly.  Yew berries are a good example of this as the seed inside the berry is highly toxic.  Birds like thrushes can eat the berry with the seed passing through their digestive system intact, so the bird comes to no harm.  If seed eating birds like finches or gallinaceous birds (pheasants, partridges etc), that have a strong grinding bill or crop, were to eat these berries they could be severely poisoned.

Special relationships

As well as these broader associations there are a number of more specific relationships between plants and animals such as the fascinating connection between Mistle Thrushes and Mistletoe.  The thrushes eat the mistletoe berries and in doing so some of the seeds inside the gelatinous fruit can get stuck around the bill.  In order to remove the sticky seeds, the thrush wipes its bill on a convenient piece of tree bark.  With luck the seed becomes lodged under a flake of bark or wedged into a damp crevice of a suitable host tree and is able to germinate.  Replicating this to try and grow your own mistletoe is notoriously difficult but I think this is largely because the thrushes do this hundreds of times until by chance a suitable location is found.  Without this association it is impossible to imagine how a mistletoe plant would ever be able to colonise a new tree. 

Another interesting relationship is between oak trees and Jays.  In the autumn, the normally secretive Jays are particularly visible as they spend much of their time stashing acorns by burying them under leaf litter and in natural holes, in the hope that later in the winter, they will be able to find them again.  It has been estimated that each Jay can bury up to 5,000 acorns each year, so it is not surprising that they don’t find them all.  You can get an idea of how important this is for the oaks when you consider that an oak tree takes around 10 years to produce its first acorns; at which time it may have a spread of up to 2.5m from the trunk.  So, without any intervention, acorns falling onto flat ground would only allow the tree to colonise new habitat at a rate of about 0.25m per year or perhaps a bit more with a lucky bounce.  From about 60,000 BC up to about 12,000 years ago, the UK was largely frozen tundra and a simplistic calculation would estimate that without assistance, oaks could only have spread about 3 km over that time period.  As oak trees are today native throughout the length of the UK, they must have spread at a rate of at least 100m per year to spread from the temperate regions of Europe where they survived, which is in the right order of magnitude for it to be explained by the activity of Jays.  One conclusion from this thought process is that all the large seeded plants (i.e. the ones that do not have wind dispersed seeds) are only present in the UK because of the distribution activities of birds and mammals (and more recently of course, humans).

This is yet another example of the complex interactions that exist in our ecosystems and the interdependency of the organisms that inhabit them.  It is also a reminder that the resilience of the natural world is reliant on maintaining the diversity of these systems and although many ecosystems have the ability to adapt and evolve to cope with slow changes in environmental conditions, rapid changes or the loss of a key species has the potential to lead to catastrophic decline and an inability to regenerate.

And on that note …..

Happy wildlife watching

Tim

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Any views or opinions represented in this blog are personal and belong solely to the author and do not represent those of people, institutions or organizations that the author may or may not be associated with in professional or personal capacity, unless explicitly stated