Supposedly, hitchhiking is illegal in California; yet, the fall season is filled with hitchhikers. You, your dog or your cat may each become unwitting accomplices in this illicit activity. A late summer or autumn hike through an open meadow, dense riparian growth or even thick chaparral will reveal these travelers looking for an easy ride. Fur filled with burrs, pant-legs covered in clinging seeds, socks painfully filled with foxtails. Fall is the time of harvest, but it is also the time for seeds– often enclosed within fruits– to search for a new destination to flourish and spread their genetic vigor.
The development of both seeds and fruits is triggered by flower fertilization. The nucleus of the fertilized egg or zygote divides and develops into the specialized components of a seed: nutrient tissue, a filament, an embryonic new plant, cotyledons and tiny roots and shoots. Fertilization also triggers hormones which cause the ovary wall to thicken and become fruit.
Fruits are an important factor in the evolutionary success of flowering plants (angiosperms) because they protect seeds, aid in dispersal, and thus play a role in seed germination. The thickened ovary wall, called the pericarp, can be separated into three layers, the exocarp, mesocarp, and endocarp, the endocarp being the inner layer of the fruit, meso in the middle, with the exocarp most external. A tomato has a soft exo, meso, and endocarp while a grapefruit has a firm exocarp with a soft meso and endocarp. Nectarines exemplify a soft exo and mesocarp with a hard endocarp. If a flower is not fertilized, it is unusual for a plant to form fruit; the unfertilized ovules will be aborted.
Three major types of fruits exist: simple, aggregate, and multiple. Simple fruits develop from one or several carpels. Aggregate fruits grow from a single flower with more than one separate carpel . These carpels develop smaller fruitlets, which cluster together, like a blackberry. Multiple fruits mature from an inflorescence– that is, a cluster of separate flowers. When the pericarp begins to thicken, the carpels fuse together and become one fruit. A pineapple is an example of a multiple fruit- each pineapple segment develops from the carpel of one flower in the inflorescence.
The sheer diversity of fruits and seeds staggers the mind, but within the marvel of biodiversity, there is also beauty. A better understanding of the different kinds of fruits and seeds can enhance our appreciation.
As with flowers, the consistency of fruit structure aids in plant identification and classification. Helpful criteria for identification may be:
– Structure of the flower from which the fruit developed
– Number of ovaries involved in fruit formation
– Number of carpels in each ovary
– Whether the pericarp is dry or fleshy
– Whether the pericarp dehisces (splits)
– If it does dehisce, how it does so
– Role of sepals or receptacles in fruit formation
Here are some different types of simple fruits with dry pericarps that dehisce:
– Legume/Pod– This type of fruit is most often seen in the Fabaceae family. Bean lupine (Lupinus spp.) pods are classic examples. These fruits dehisce along two sutures.
– Follicle– California milkweed (Asclepias californica) seedpods illustrate this fruit, which forms from a single carpel and splits along a single suture.
– Capsule– Capsules arise from compound ovaries from flowering plants such as Yucca whipplei or Datura wrightii.
– Silique– Plants in the family Brassicaceae, such as milkmaids (Cardamine californica) produce this dry fruit pericarp that separates into three portions. The seeds are attached to the central persistent portion of the fruit.
Some simple fruits with dry pericarps that do not dehisce include:
– Grain– Species in Paoaceae (grass family) form fruits that are grains such as rye, corn, or wheat or can be seen in native grasses such as Douglas blue grass (Poa douglassii.)
– Samara– This fruit can be identified by the “wing” like structures growing out from the ovary wall. Velvet ash (Fraxinus velutina) and box elder (Acer negundo) develop samaras.
– Schizocarp– Fruits from the Apiaceae (carrot family) such as cow parsnip (Heracleum lanatum) illustrate schizocarps. These segmented fruits break up into one-carpel segments called mericarps. Each mericarp has one seed.
– Nut– A nut is a hard-shelled indehiscent fruit such as a California black walnut, acorn or hazelnut.
Some simple fruits with fleshy pericarps are:
– Drupe– Cherries, peaches and olives exemplify these usually one-seeded, single carpel fleshy fruits. California’s holly leaf cherry (Prunus ilicifolia) forms drupes each fall.
– Berry– Fruits developing from compound ovaries are berries, such as tomatoes, watermelon, cucumbers, or some nightshade fruits such as Solanum umbelliferum. Citrus fruits are a specialized berry called hesperidium.
– Pome– A pome is composed of one or more carpels (often five) surrounded by fleshy tissue. The central carpels form the “core” which is common in pears and apples.
Given that plants are found growing in radically different habitats– from watery swamps to windblown deserts to dense forests– it only makes sense that seeds and fruits mirror this diversity. For it is this wild array of fruit and seed design that allows seeds to travel, utilizing multiple modes of transportation.
Seeds and fruits are adapted to take advantage of the best methods of seed dispersal that their habitat provides. For example, to benefit from gentle breezes and strong winds, many seeds have grown specialized appendages to ride the air currents. Feathery plumes, as seen in the seeds of a dandelion or the wings on a samara, exemplify adaptations for seed dispersal by wind. Violent dehiscence is another mode of wind dispersal. The sutures of a milkweed pod burst and the seeds’ silky appendages explode out to catch the wind and spread genetic material.
Water also serves to disperse seeds and fruits. Seeds surrounded in a membranous envelope filled with air ensure that the seeds will float to a new destination. Sedge seeds take advantage of this strategy. A hard, fibrous exocarp, as seen on a coconut, offers a different possibility for water travel.
An alternate seed dispersal tactic employs both fruits and animals. Ingested fruits, seed included, pass through the alimentary canal and are deposited in new locations with their own fertilizer. Examination of animal scat reveals a new world: not only can you see what the animal has been eating but you can elucidate what fruits are ripe and where that animal has been feeding. Some seeds have developed a sticky coat. As with mistletoe, when birds eat their berries, the seeds stick to bird’s feet. When this bird flies to a new destination, mistletoe seeds find home in a new location.
Humans are great seed dispersers, intentionally planting seeds for food, landscaping, habitat recovery and conservation. But other mammals and birds also plant seeds. Squirrels and jays are oak trees’ greatest advocates. When storing food for winter, these animals cache acorns underground. Within one fall season, one squirrel buries hundreds of acorns. Thankfully they do not recover them all.
Barbs, hooks, and spines are yet another adaptation of seed dispersal. You may have picked up a barbed cocklebur and launched it at your friend only to see its momentum immediately halted as it stuck fast to your friend’s clothing. Or perhaps you have sat down in a clover field where only months ago you marveled at its luxurious softness. But now, in the fall, this same field renders painful stabs and palms filled with spiky burrs. The seeds stick to anything in order to be relocated and you have become an unwitting vehicle for their travel.
Sometimes these hitchhikers can be a nuisance, exploiting our soft clothing, our moments of relaxation. But there is ingeniousness in their passive collaborative strategy. In fact, all of seed dispersal is startling in simplicity and effectiveness. Plants have devised remarkable energy saving strategies for their transport. Not only is their dispersal efficient, it is beautiful. Different shapes, sizes, colors, tastes, smells, and textures are but a few of the artistic details that fill our senses. As we harvest the bounty of fruits and vegetables this fall, I wonder what other ingenuities we can harvest from the plants. Efficiency, collaboration, and beauty?