With autumn just around the corner, a shift is occurring in the reproductive stage of trees – the unique flowers that have consumed spring and summer are becoming seeds! Seeds are a principal means of reproduction for angiosperms and gymnosperms. The production of seeds is often the result of sexual reproduction in plants, although some plants are self compatible. Self compatible plants are individuals that are able to fertilize themselves, meaning the individual can self-pollinate.

Seeds serve four main functions which pertain to reproduction, dispersal, protection and replication. Some of these functions are seemingly straightforward: expanding genetic diversity, providing replacement plants, and increasing plant vigor over time. Other seed functions are more abstract. A seed must also be able to support radical emergence and germination through endosperm reserves, respond to stimuli like fire or digestion, and even sustain itself through dormancy periods.

Structures of a seed, A, bean seed, B. Corn seed, C. Pine seed

Photo credit: seedparade.co.uk

 

Fruit and Subsequent Seed Classification of Angiosperms and Gymnosperms

Below describes fruit and seeds types you will commonly encounter in the Chesapeake Watershed. As a quick refresher, angiosperms are plants most recently evolved within the plant kingdom, appearing on Earth approximately 140 million years ago. These plants produce flowers, fruiting bodies and seeds. Gymnosperms, appearing on Earth approximately 320 million years ago, are plants that maintain “naked seeds,” meaning the species’ seeds lack ovaries or fruit.

As you move through the classification information below, some plant jargon may stand out to you. The terms dry and fleshy refer to the status of the pericarp. A pericarp is the wall of a ripened fruit, essentially a mature ovary that acts to protect seed development. If a pericarp is dry, it lacks moisture at maturity. If a pericarp is fleshy, it maintains moisture at maturity, so the fruit wall will be composed of a pulpy, fleshy tissue. The word dehiscent refers to the act of the pericarp splitting open at maturity; indehiscent pericarps will not split open at the time of maturity.

 

Angiosperms: fruits and seeds derived from a single flower

Dry, dehiscent

  • Capsule: A fruit that forms from two or more fused carpels (a term describing the female reproductive organs of a flower). Generally, the fruit will split along greater than two sutures.
two closeups of spiky light green plants

Hamamelis virginiana (witch hazel, left) and Liquidambar styraciflua (sweet gum, right) is a capsule of multiples. Seeds are released from fruits that split open along many seams. Photo credit: thefieldguidespodcast.com, floridaseeds.net

  • Legume: Seeds are released from this fruit by the splitting of two sutures or seams. Developed from a single carpel.
Two closeups of seed pods on tree branches

Gleditsia triacanthos (honey locust, left) and Robinia pseudoacacia (black locust, right). Two distinct seams are visible on either side of the legume pod of honey locust and black locust. Photo credit: amkhaseed.com, minnesotaforestry.org

  • Follicle: Seeds are released from this fruit by splitting on one suture or seam, derived from a single carpel.
A closeup of a row of red seeds next to a closeup of a fluffy white seed pod

Magnolia acuminatat (cucumber magnolia, left) is an aggregate of follicles. The Asclepias Syriaca (milkweed, right) pod is split along one seam. Photo credit: hikinginthesmokies.wordpress.com, naturegardenlife.com

 

Dry, indehiscent

  • Achene: A multi-seeded fruit in which each seed is attached to a thin pericarp at only one point.
Two closeups of spherical fuzzy plants

Platanus occidentalis (American sycamore, left) and Clematis virginiana (virgin’s bower, right) are two examples of achenes. Photo credit: naturewalk.yale.edu, gardenia.net

  • Nut: A one-seeded fruit usually with a woody or leathery pericarp, or encased in a husk.
Two closeups of acorns and hickory nuts on tree branches

Quercus sp. (oaks, left) and Carya sp. (hickories, right) nut seeds. The green leathery pericarp is apparent on the hickory nut. Photo credit: mossyoak.com, mossyoak.com

  • Samara: A one-seeded pericarp with “winged” appendages.
a closeup of long brown seed pods (left) and propeller-like, light red seed pods (right)

Aggregate samaras of Liriodendron tulipifera L. (tulip poplar, left) and Acer rubrum (red maple, right) samaras. Acer sp. (maples) maintain fused samaras. Photo credit: commons.wikimedia.org, Rodd Halstead

 

Fleshy

  • Berry: The pericarp maintains a fleshy or pulpy mass that contains one or more seeds.
a closeup of red berries next to a closeup of blue berries

Diospyros virginiana (persimmon, left) and Vaccinium angustifolium (lowbush blueberry, right) are examples of berry fruits. Photo credit: mgnv.org, gardeningknowhow.com

  • Drupe: A one-seeded fruit in which the seed is not separated from the pericarp via a a papery wall.
two closeups of different plants with round, blue fruits on the branches

Nyssa sylvatica (black gum, left) and Prunus serotina (black cherry, right) seeds are both drupes as they maintain one seed. Photo credit: floridaseeds.net, amkhaseed.com

  • Pome: A multi-seeded fruit in which the seed is separated from the pericarp via a papery inner wall.
A close up of red fruit next to a closeup of blue/red fruit on plant branches

Crataegus sp. (hawthorns, left) and Amelanchier canadensis (serviceberry, right) species are pomes, multi-seeded and have a papery inner lining separating flesh from seed. Photo credit: minnesotawildflowers.info, green-weaver.com

  • Hesperidium: A multi-seeded fruit with radial sections. It maintains a leathery exterior and generally a fluid filled interior, most notably in the Rue (Rutaceae) Citrus Family.

 

Fruits and seeds derived from an inflorescence

Dry, dehiscent

  • Strobile: A cone-like fruit developing from catkins.
a closeup of multiple pine cones next to a closeup of a lighter, white cone

Alnus sp. (alders, left) and Betula sp. (birches, right) strobile are two common examples of dry, dehiscent fruit types you can find in riparian zones. Photo credit: plantwiseplusknowledgebank.org, friendsofeloisebutler.org

There are also fleshy fruits and seeds derived from inflorescences that are called Syconium, Sorosis and Coenocarp fruits. These fruit types are far less common in PA forests, so they will not be discussed in greater detail.

 

Gymnosperms: plants that maintain “naked seeds”

Dry Strobili

  • Cone: A woody structure that is typically formed by an arrangement of scales. It will generally open while attached to the tree to disperse its seeds.
two closeups of different species of pine cones

Tsuga canadensis (eastern hemlock, left) and Pinus strobus (white pine, right) pine cones. Photo credit: picturethisai.com, davey.com

 

Fleshy Strobili

  • Drupelike: A cone with fleshy merged scales, it can enclose a single or multiple seeds.
plant branches with many light blue round fruit on them

Juniperus virginiana (Eastern red cedar) berries are not true berries. Photo credit: eattheplanet.org

 

Dispersal Mechanisms

Now that we have an understanding of types of fruits and subsequent seeds that are present in our landscapes, let’s discuss the mechanisms in which seeds can be dispersed!

Seeds can be dispersed by a variety of methods that are dependent on a number of factors. Some of those factors include the environment that a species inhabits, the general anatomy of a seed, or even how palatable the seed is to the local fauna.

The first dispersal method to consider is simply gravity! This mechanism is known as barochory. Imagine a seed falling to the ground- perhaps it sits in place, bounces, or maybe it rolls down a hill. Whatever path the seed follows is due to gravity. Barochory is common in Quercus sp. (oak) acorns, Carya sp. (hickory) nuts and other hardened seeds.

Anemochory is the dispersal mechanism relating to wind. Wind is fairly ubiquitous to all environments. Because of this, many seeds (regardless of if they maintain anatomy specifically designed to be vectored by wind) are affected by wind. Seeds that maintain anatomy for optimizing wind dispersal are those with wing-like appendages and/or hairs. For example, Asclepias sp. (milkweeds) are borne with white fluffy hairs that you can readily see flying around in late summer to fall. Acer sp. (maples) and Fraxinus sp. (ashes) maintain wings to help in increasing projection potential.

a closeup of fluffy seeds been blown by the wind

Wind vectored Asclepias (milkweed) seeds with follicle pod. Photo credit: istockphoto.com

Hydrochory is the passive process of water acting as a vector for seeds. Seeds dispersed by water usually maintain hairs or another anatomical trait, like Alnus sp. (alder) seeds that are flattened and light weight, to assist in buoyancy. These species are often found growing in riparian zones.

Zoochory, or dispersal of a seed by fauna, is a more complex vector mechanism. Seeds can easily hitch-hike on the fur of mammals, depending on the nature of the seeds anatomy. If you consider a seed with hairs, other projections, or a sticky/sappy residue, attachment to fur for any distance is possible. Seeds consumed by an animal and subsequently eliminated from the animal are also considered to have zoochory vectoring. Berries and other high value seeds are often vectored through animal digestion (an example of this would be Diospyros virginiana, persimmon). Zoochory also relates to animals that cache their food. If an animal creates a cache, then forgets its location or for another reason doesn’t make it back to its cache those seeds were dispersed by zoochory. Quercus sp. (oaks) and Carya sp. (hickory) nuts and acorns are often cached.

Seeds can also be dispersed through a method called ballochory. Ballochory relates to ballistic dispersal. Essentially as a plants seed head dehisces, tension builds up and an explosion or ejection of seeds will occur. This catapulting of seeds can be active or passive, meaning seeds can be ejected by an event or arbitrarily. A few species that maintain ballochory are jewelweed (Impatiens sp.) and witch hazel (Hamamelis virginiana), which has been reported to eject seeds at speeds up to 30 miles per hour!

Anthropochory seed vectoring relates to humans acting as the mechanism for dispersal. This dispersal mechanism is extremely diverse! Seeds can attach to articles of clothing, people may actively plant seeds from one place to another, seeds can be transferred through agricultural practices and even transported in ballast water!

Now that you have a brief overview of seeds, the function they serve, general type classification and some ideas of dispersal mechanisms, go out and explore this reproductive cycle of our trees!

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