On Angiosperms

By Andrea Macdonald

"It must have been around seven am when a small shrew like creature stumbled out of its dark burrow and peered near-sightedly at the first flower, and decided it wasn’t dangerous. "
This is a line from the poem “in the early Cretaceous” by Al Purdy. The poem goes on to fancifully describe the proliferation of flowering plants during the Cretaceous. Reading the poem one can’t help but wonder what it was really like during the Cretaceous, how did those first few flowers come to be? How rapidly did they develop into a dominant form of flora and what made them so successful?
At the beginning of the Cretaceous the earth was warm and wet. There were no glaciers at the poles and the continents were grouped closely together. Dinosaurs and gymnosperms were the dominant fauna and flora. By the end, the continents had dispersed, dinosaurs had nearly become extinct, mammals were on the rise, and angiosperms had replaced gymnosperms as the dominant plant life.

So what came first, the Carpel or the ovary? That is, how did angiosperms evolve from gymnosperms? The main difference is in the reproductive tissues. Gymnosperm ovules occur on sporophylls, that is, open leaves. Angiosperms differ in that they have many different tissues surrounding the ovule; there is the ovary which is at the base of the carpel, there are the stamens, the petals, and the sepals. It is believed that these structures all evolved from leaves. Once upon a time a sporophyll began to curl around the ovules until it became fused and a carpel was formed from the fused leaf. That shrew could have stumbled out and seen a whorl of leaves with shortened internodes before he noticed the first flower. This bundle of leaves with a rolled leaf surrounding an ovule might have initially been an evolutionary advantage because the seed was better protected from the elements and/or seed eating fauna by the group of leaves.
The Amborella supports the fused sporophyll theory. Amborella is believed to be the most primitive flower that still exists today (based on molecular data). The Amborella has a carpel that is not fused but is held together by secretions, a possible evolutionary step leading to the fused carpel.
Somehow this bundle of leaves became attractive to insects and from there, more and more attractive and colourful species emerged in order to be more attractive to pollinators. (Some wind pollinated angiosperms did not develop these bright colours.) Most gymnosperms rely on wind to disperse their seeds, while angiosperms take advantage of a symbiotic relationship with insects and birds (possibly their most important evolutionary advantage) , in which the animal is rewarded with a nectar for dispersing the genetic info (pollen) of the flower. How the first bee ancestor became attracted to flowers is a mystery, but it is generally accepted that the success of angiosperms is closely related to their relationship with insects and birds.
By the early cretaceous wasps and moths had evolved and during the cretaceous winged dinosaurs and some birds were becoming successful species. Perhaps this was a disadvantage for the gymnosperms, which keep there maturing seeds up high possibly as a way to keep them safe from herbivores. Angiosperms on the other hand took advantage of flying creatures not only to aid in pollination but also to help in seed dispersal. Many angiosperms produce seeds that are meant to travel through the digestive system of an animal before they are meant to germinate. Angiosperms are the main food source of mammals and we have evolved and diversified simultaneously since the mid Cretaceous through the process of co evolution.
One example of this co evolution is how insects aided in the speciation of angiosperms. Insects provide an alternative method of pollination to wind. They allow for wide dispersal of genetic material which leads to more opportunities for mixture of genetic material. Because one bee may visit many different flowers, pollen obtained containing a mutation would be dispersed widely. The plants resulting from fertilization by the mutant pollen may produce mutated flowers which might not attract the original pollinating species and so the newly mutated species would not be cross pollinated with the parent species. This creates the sexual isolation necessary for speciation. The short lifecycle of angiosperms also allowed them to speciate rapidly. More variety of species means greater chance for species to be suited to difficult environments.
Fossil evidence of flowering plants suggests that they were first successful in environments where it was difficult for gymnosperms to survive. Riverbanks and areas that experienced unstable environments were dominated by angiosperms millions of years before they began to compete with gymnosperms for space in more stable areas. Their short lifecycle also aided in this success. Angiosperms can go from a seed to a mature adult in a matter of weeks or months whereas gymnosperms take over a year to develop a seed. Angiosperms also developed a more efficient form of vascular tissue which would have helped. In addition to the same xylem and trachieds of gymnosperms they also have vascular vessels for transporting water up the stalk.
Interestingly, Amborella does not include these vascular vessels suggesting it diverged from the other angiosperms prior to this development.

Fused leaves, rapid speciation, symbiotic relationships with birds and insects, suitability to harsh climates; these are some of the main ideas about how and why angiosperms evolved. There are entire books and university courses devoted to this topic I found out in my research. Darwin referred to the appearance of angiosperms in the fossil record as “an abominable mystery.” So many years later much of the mystery remains, a vast deal of research has been done and the theories proliferate like dandelions.
As Al Purdy wrote, “no one can ever know what it was like, that first time on Primordial Earth/ when bees went mad with pollen fever and seeds drifted away from home on little white parachutes without a word to their parents/ no one can ever know, not even when someone is given the gift of a single rose and behind that rose are the ancestors of all roses and all flowers and all spring times for a 100 million years of summer, and in her eyes an echo of the first tenderness…”

References:
Biology sixth edition, Authors: Neil A. Campbell, Jane B. Reece. Copyright 2002, Benjamin Cummings
The Historical Atlas of the Earth, Authors: Stephen J. Gould, Roger Osbourne, Donald Tarling. Copyright 1996, Henry Holt and Company.

Angiosperm Origins and Evolution
Copyright © May, 2001by: Sebastian Molnar
http://www.geocities.com/we_evolve/Plants/angiosperm.html

Angiosperms
Flowering PlantsPam Soltis, Doug Soltis, and Christine Edwards
http://www.tolweb.org/Angiosperms