How are the fruits and seeds of plants dispersed?

In the collective imagination, plants do not move, they are rooted in the soil. But then, how do they manage to reach new territories? In reality, it is not they that travel, but their offspring: the seeds and fruits. Through their dispersion, these structures can reach distances ranging from a few meters to large geographical areas. This process constitutes one of the great evolutionary innovations of the plant kingdom and has allowed vascular plants to colonize a large part of the planet. Today, there are families and genera present on almost all continents and terrestrial environments. Neither oceans nor large mountain ranges have always been insurmountable barriers.

This way of “moving” without moving directly is part of the dispersal strategies of plants. This involves two main mechanisms: pollination, which allows the gametes to meet, and the dispersal of seeds, which helps them move away from the mother plant. To make dispersal possible, the plant provides the embryo with nutritional reserves and surrounds it with protective layers to resist adverse conditions: this is what we know as a seed. It is, in a way, a 'suitcase' with everything necessary for the embryo to survive during transport and begin its development in another place. In many species, this seed is even more protected within an additional structure: the fruit. It often adopts fleshy, attractive or resistant shapes that facilitate dispersal. For example, in a cherry, the fleshy part of the fruit acts as a vehicle that helps transport the seed that is inside.

Once formed, seeds and fruits begin a real journey. Some float across the oceans, others glide with the wind, many travel thanks to animals and, there are even some that 'explode' to project their offspring far from the mother plant.

Some seeds and fruits travel through rivers, lakes, and seas thanks to structures that allow them to float and withstand immersion for varying periods. An example is the coconut fruit ( Cocos nucifera ) that grows on tropical coastal margins. The fruit has a nutrient-rich interior and a fibrous, lightweight outer covering that helps it float as it is carried over long distances by ocean currents.

The wind is another ally of plant dispersal. In this case, the seeds and fruits are very light; they often have structures that increase their surface area and reduce the speed of fall, facilitating aerial movement. A well-known example is the dandelion ( Taraxacum officinale ), very common in spring and easily recognizable by its dry fruits -called achenes- with a plume of hairs called pappus or vil·là. Popularly, they are known as “buffs” or “angelets” and some people blow them while making a wish. Other seeds that are carried by the wind over long distances are those of the poplar (Populus), which are surrounded by cottony hairs that form the typical spring “cotton” flakes.

Fish, birds, ants and rodents are some of the animals that disperse seeds. This mechanism is very diverse and each animal has a different type of interaction.

In some cases, animals eat the fruits and later expel the seeds elsewhere: this is internal dispersal or endozoochory. An example is the yew (Taxus baccata), a tree with potentially high dispersal since its fruits are food for birds, rodents, and other small mammals, which eat them and at the same time help transport the seeds.

In others, the seeds simply stick to the fur of mammals, the feathers of birds or even our clothes and travel without us noticing: this is external dispersal or ectozoochory. A classic example of this mechanism is the burdock (Arctium lappa), with fruits equipped with hooks that easily stick to the fur of animals, facilitating passive transport over long distances.

There are also seeds that carry small nutritious structures capable of attracting insects, which transport them to their nests or shelters -myrmecochory-. This is the case of ants, which transport the seeds to their nests, thus contributing to their short-distance dispersal.

Sometimes, the plant does not need the help of any external agent, but uses its own mechanisms to disperse its seeds. They do this during the ripening process, where the fruits accumulate mechanical stresses and, at a certain moment, they break suddenly, projecting the seeds at a distance. This mechanism is typical of legumes or fabaceae, where the pods dry unevenly and pressures are generated that heat up and cause them to open abruptly: an explosion that disperses the seeds. In Catalonia, the broom (Spartium junceum) or wild geraniums (Erodium spp.) are the best known cases.

The diversity of shapes and structures is an intrinsic fact of nature, and the case of plants is no exception. In some species, the same plant produces different types of fruits with different dispersal strategies and, consequently, the place and conditions in which they can establish themselves in a territory will also be different. This is known as heterocarpy: usually, some fruits have a greater capacity for long-distance dispersal, either by the wind or by sticking to the fur of animals, while others remain close to the mother plant. This diversity allows for the combination of the exploration of new habitats and, at the same time, maintaining the population in already favorable environments.

One case is Leontodon longirostris . This plant has fruits called achenes – small nuts with a single seed inside – of two different types. The central ones have a tuft of hairs that facilitate dispersal by the wind, while the outer ones are thicker, without a tuft of hairs, and tend to stay close to the mother plant. They also germinate differently: the former can germinate more quickly when conditions are favorable, while the latter can remain in the soil longer. This strategy staggers germination and increases the chances of survival.

This combination of different types of fruit is especially useful in variable environments, such as the Mediterranean, where conditions can change greatly from one year to the next. Not relying on a single dispersal or establishment strategy increases the chances that at least some of the offspring will be successful. This environmental variability can generate differences in the proportion of both types of fruit, as has been observed in the Iberian populations of Leontodon longirostris studied in the POREXPAN project, led by CREAF researcher Maria Mayol. The results show that populations on the expansion fronts -located in the north- have a higher proportion of fruits with high dispersal capacity, while in the south the number of fruits that remain close to the mother plant increases.

Variation within a single species is not limited to the type of fruit or its proportions, but can also appear in the thickness and composition of the tissues surrounding the embryo. These tissues condition the entry of water and gases and, therefore, influence the timing and dynamics of germination. These differences can be adaptive, since it is not the same to germinate in humid and temperate environments as to do so in arid areas or in places where frost can put the survival of seedlings - young plants that have just germinated - at risk. In this sense, natural selection can favor tissue configurations that allow rapid germination when conditions are favorable or, on the contrary, that delay germination until the environment is more suitable.

This relationship between fruit structure and environmental conditions is precisely what has been observed in the species Leontodon longirostris within the framework of the POREXPAN project. In populations of the Iberian Peninsula there is variation in the size and proportion of tissues surrounding the embryo within each type of fruit (central and external), depending on the north-south climatic gradient, and these differences are associated with the germination rate.

Once a seed is dispersed, the work is not over: it still needs to be able to germinate, take root and grow. If conditions are not right or there is too much competition, the new individual will not thrive. Ultimately, dispersal is only effective when it is accompanied by good establishment of the new individuals. Both dispersal and establishment depend on environmental conditions, which are changing in many places due to human action and climate change.

One of the most important effects of human action is habitat fragmentation: when the continuity of the landscape is broken by infrastructures, such as roads or cities, plant populations are divided into small fragments surrounded by areas that are not very favorable for the establishment of new individuals. In this context, dispersing seeds and fruits too far can be a disadvantage, since it increases the probability that the seeds end up in places where they cannot survive. This could favor, in the long term, seeds that disperse over shorter distances and remain within favorable habitats. This process was studied in several asteraceae in the PLANT DISPERSAL project.

In the case of the species Mycelis muralis , the results showed that plants from more isolated populations had a lower dispersal capacity, both at local and regional scales. These results indicate that landscape fragmentation not only affects the distribution of plants, but also the evolution of their dispersal capacity.

Climate change can also affect the ability to colonize new territories. An example is the yew (Taxus baccata): although its seeds can be dispersed over long distances thanks to birds and small mammals that ingest the fruits, rising temperatures and persistent droughts in the Mediterranean are making it increasingly difficult for new plants to establish and survive.