Lucy Bennison Laffitte, MEd, PhD
Executive Committee, International Big History Association
North Carolina, USA
One of the ways to think like a Big Historian is to use the concepts that describe phenomena from one sector – cosmos, Earth, life, or humanity – as metaphors to understand phenomena in another sector. For instance, when we say that stars are born, develop over time, and die, we’re borrowing terms (birth, development, death) from the LIFE sector, and using them as metaphors to help us understand the COSMIC sector. This transdisciplinary method—using categories from one sector as metaphors to understand phenomena in another sector—is thinking like a Big Historian. It’s a literary method applied to scientific data, and unique to the field of Big History. It makes human ways of knowing explicit in our understanding of science, and is a very powerful tool for citizens of the twenty-first century.
To illustrate how useful Big History thinking can be, I want to clarify the baffling morphology of land plants, answering this question: Why do some plants have cones, others spores and thalli, and still others flowers? In this case, I’ll be working within the LIFE sector, but I’ll be using the life history categories of land animals as metaphors to understand the life history of land plants. Hang with me.
Evolutionary Challenges and Responses
Two important evolutionary challenges emerged for plants and animals as they crawled out of the water and onto land:
- How do sperm swim to egg? Remember: Sex evolved in water and became dependent upon a watery medium to consummate fertilization.
- How is the developing embryo cared for, and then dispersed from home? The young find “room and board” in water and can float away from their parents’ territory in currents. Not the case on land.
Let’s delve into how animals solved these two problems. Vertebrate land animals evolved from fish to amphibians, from amphibians to reptiles, and from reptiles to mammals. (The more we learn about birds, the more we realize they are essentially flying reptiles.)
- Amphibians solve the problem of getting sperm to egg by returning to the water for reproduction. The male releases sperm in the water so they can swim to the eggs laid there by the female. Amphibians solve the problem of providing the developing embryo with “room and board” by keeping the young in water, separating them physically from the adults. Adults live and feed on land while the young do so as water-borne tadpoles or larvae.
- Reptiles solve the problem of getting sperm to egg by using internal fertilization. The male releases the sperm in the moist environment inside the female. Reptiles solve the problem of providing the developing embryo with “room and board” with a leathery egg buried in the moist ground in an area away from home.
- Mammals solve the problem of getting sperm to egg in the same way as reptiles, with internal fertilization. They solve the problem of providing care for the developing embryo by retaining the young inside the body of the female, which ensures a temperature-controlled home and a steady supply of food and oxygen-enriched blood. Mammals also extend the length of time the young are cared for, suckling them in built nests and providing lessons on “culture.”
So how can this help us understand our opening question—why do plants have spores, thalli, cones, or flowers?
Applying Big History Thinking to Plant Sex
Using Big History thinking, we can turn the three terms—amphibian, reptile, and mammal—into metaphorical categories to better understand plants: the amphibian-like plants, the reptile-like plants, and the mammal-like plants. Let’s use these metaphorical categories to understand how sperm gets to egg and the developing embryo is cared for in plants.
The amphibian-like plants include the mosses and ferns. Mosses include an assembly of short, primitive land-plants called liverworts and hornworts. Ferns include lycopods, horse tails, and whisk ferns. These plants share the amphibian-like life style, conducting their sexual activity in the wet splash zone. During the Carboniferous period, these plants grew to the size of trees. So how do tree-size plants have sex in the splash zone? With the “invention” of spores and a specialized organ called a thallus, of course.
Mosses and ferns use the wind to drop spores to the splash zone. In the presence of water, the spores grow into a separate structure called a thallus. It’s on the thallus that the antheridia (testes) produce the sperm that swims to the egg in the archegonia (ovary). Fertilization creates an embryo, which sprouts on the rhizoid-fed thallus. Thus, in mosses and ferns, there is some care of the developing young. In the fern group, the tallest of these amphibian-like plants, the thallus disintegrates as the embryo matures into a full-grown fern.
The reptile-like plants, called conifers (cone-bearing plants), have figured out how to have sex without water; they do it with cones. Conifers have two different types of cones—male and female. Male cones are staminate, and contain sperm. Female cones are ovulate, and contain eggs.
How do cones, up high in a tall tree in the middle of a desert, help conifers get sperm to egg? The staminate cones release thousands and thousands of tiny, four-cell sperm in a fluted, wind-loving pollen grain. The ovulate cones contain scores of layered, prickly, pollen-collecting scales that crack open to let the pollen in. Each scale contains two winged ovules, side by side. Each ovule contains the unfertilized egg, which is surrounded by a water-tight seed coat. When a pollen grain gets caught in an open scale, the pollen grows a tube of tissue that delivers the sperm in its tip precisely to the opening of the coated egg. Sensing successful fertilization, the scales close tight, protecting the developing embryos in a bulky, inedible cone for a year or two. When the embryos are ready, the cone scales reopen and send the winged seed twirling through the wind, far from home. The seed coat keeps the embryo alive for years. The seed itself contains a juicy morsel of food for germination, prepared by the mother ahead of time. In this way, conifers use cones to get sperm to egg without water and to care for the embryo during development and germination.
Mammal-like plants, called flowering plants, care for the developing embryo during development and germination in the same way as conifers. Flowering plants, however, have figured out how to get sperm to egg, and then disperse the young great distances from home. They nurture their young long into the future by ingeniously enlisting the aid of helper animals. This might be the greatest symbiotic story of all time.
Flowering plants (for the most part) do not rely on the inconstant wind to deliver sperm to egg. Instead, they’ve “hired” one delivery service to carry the pollen grains to precisely the right place for a pollen tube to hand-carry the sperm to the egg. These mammal-like plants have hired yet another delivery service to pick up the developing embryo–which remains safe inside a watertight seed–and drop it off miles from home in a pile of nutritious fertilizer. These plants engage the first delivery service with colorful landing platforms, fragrant scents, easy-to-understand directions, and edible rewards. In other words, they build a flower. They engage the second delivery system with bright nuggets of fruit flesh or nutmeats. Flowers allure specific insect pollinators to follow meticulous delivery instructions, and then reward them with a payment. Flower fruits allure animal dispersal agents to ingest seeds and deposit them in piles of defecated fertilizer. Flowering plants are the mammals of the plant world, facilitating sperm to egg and long-term care of the young. And all of this is done completely independent of water.
Big History Thinking: A Powerful Tool
As these examples demonstrate, Big History thinking offers us a powerful tool for understanding the vast complexity that has emerged since the Big Bang. With the simplified categories of amphibian-like, reptile-like, and mammal-like, we can understand and even remember why plants bothered to evolve such odd structures as spores, thalli, cones, and flowers. Take some time to study the figures below, tracking the morphology of the corollary structures in plants and animals as they solved the problems of moving from water to dry land.
How animals and plants transitioned from water to land: Getting sperm to egg. Click to enlarge.
How animals got from water to land: Protecting the developing embryo. Click to enlarge.
About the author: Lucy Bennison Laffitte, MEd, PhD, has been teaching science in-context for over 30 years. She has taught in the field, in the classroom, and online. She has published in print, on air, and on the web—authoring a newspaper column, founding an award-winning environmental radio program, creating certificate programs, and developing digital learning objects for public television.