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November, 2016

The Shapes of Seeds

by Valerie

The seeds of plants are marvelous things. In mundane terms, a seed is an embryonic plant covered with a protective coating. Everything needed to create a shrub, vine or herb is enclosed in a convenient little parcel - just add water and the magic of growth begins. Of course, there are a few more ingredients needed to eventually obtain a healthy and mature plant. Sunlight, soil, bacteria, fungi and warmth come to mind. With the basic components, though, objects that look more like gravel than living things manage to become plants.

Seeds, structures attached to them, and the pods that hold them, come in an astounding array of shapes. Some seeds are so distinctive that they give a plant its common name. Devil's claw, wafer ash, cottonwood, beggar's-ticks, snailseed and coconut all have seeds more distinctive than their leaves or flowers. As with many biological features, the form suits the purpose: seeds need to be protected and also get to a place where conditions are good for growth. A species has the best chance of producing descendants if those progeny can disperse to new areas where they can then continue to multiply.

It is arguable that we would not be the dominant and successful species we are if it were not for seeds. Our most important foods, responsible for sustaining billions as the core of their diet, are, quite literally, grass seeds. Our major grain crops, rice, corn and wheat, are certainly not grown for their leaves or roots. It is the seed that becomes our flour, dough and bread.

Besides the important nutritional seeds, there are those that we love because of their taste or other qualities. Sprinklings of poppy, sesame, sunflower, caraway, and celery seeds add flavor to breads and salads. Almonds, cashews, pumpkin seeds, pecans and soybeans are some of my favorite snacks. Peanuts are practically a staple food if you enjoy peanut butter sandwiches. Pine nuts, macadamia nuts and pistachios are some of the most expensive (but oh, so tasty!) foods. Vanilla, an ubiquitous flavoring, totally delicious but so common that it is called "plain," comes from the seed pod of an orchid. Finally, civilization wouldn't be the same without two extraordinarily important beans: cacao and coffee.

After noting all the dietary contributions, it might seem an effort to return to the main purpose of seeds, which is plant reproduction. Since plants are the whole basis of our food chain, making life as we know it possible, their continuing success is a rather major concern of ours. But we can also just appreciate the myriad ways in which relatively stationary biological beings have evolved to spread their progeny to new habitats. Making use of wind (anemochory), water (hydrochory) and animals (zoochory), the seeds of plants can travel with an efficiency that is the envy of any savvy globetrotting tourist.

Plants that have very narrow habitat requirements do better to restrain their travel from the parent plant. Very heavy seeds will just fall to the ground, where they will follow the rule "what was good enough for mom is good enough for me." If they are smooth and round, they might roll a bit. Rain is an excellent medium to dislodge seeds, as it is also providing the necessary moisture needed for germination. The splashing of water droplets can also carry seeds a short distance away; just far enough to leave home, but not get into trouble. Some seeds need no outside help (autochory) and just go ballistic, with spring-loaded launching mechanisms built into the pod that shoot the seeds out as they dry or by the use of hydraulic pressure to spray the seeds. If you want to check this out, touch several seed pods on the common yellow wood sorrel (Oxalis stricta). When you hit one that is ripe, the seeds inside will pop with a force that sends them several feet away. Some seeds are capable of geocarpy, or self-burial. Our local spear grass (Nassella leucotricha) has horribly sharp seeds with tails that twist as they dry. They not only bury themselves into the soil (very beneficial for the plant) but they can get tangled in fur or socks and perform the same maneuvers, burying themselves into the underlying flesh. People can easily feel the first prick and remove the offending seed with their fingers, but dogs sometimes have a hard time pulling them from their coat and the seeds will penetrate well into their skin. Surprisingly, this grass species is sometimes sold in commercial native grass seed mixes under the benign name of Texas winter grass. Buyer beware!

The sheer variety of seeds makes them interesting. They are like works of art, but those fantastical forms follow function. Wings, plumes, and gossamer threads carry lightweight seeds on the breeze. Barbs, spines, glue and, yes, the hooks that provided the inspiration for Velcro, enable seeds to attach to mobile animals long enough to reach new territories. We might find burs and stick-tights annoying, but they are essential survival tools for some plants. Fruits, the discussion of which would be another entire essay, are vessels whose purpose is to lure animals into eating them, carrying the seeds through their gut and depositing them, often along with fertilizer, in widespread areas. Even flowers, so welcome to our color-hungry eyes and beauty-seeking sensibilities, are a development that evolved in plants to improve their chances of producing healthy seeds.

As often happens when I study some aspect of my surroundings, understanding the biological basis for any structure is just the beginning to a deeper appreciation for what I find in nature. As a kid, I played with seeds of various kinds, smashed open countless walnuts and hickory nuts to eat the luscious flesh inside, and watched in wonder as a watered seed in a pot sprouted and grew. Who hasn't thrown maple seeds into the air to see them spiral downwards like miniature helicopters, blown on milkweed seeds to enjoy their iridescent gossamer carry them away in the wind or admired the exotic sea-beans washed up on our eastern beaches? With textures from glossy to matte to dimpled to spiky, and colors ranging from white to brown to red, as well as a variety of forms that seem endless, seeds are one more reason to admire and marvel at the world around us.

A Plethora of Seeds and Pods

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