Chapter 29, Problem 29.1CR

Draw a phylogenetic tree illustrating our current understanding of plant phylogeny; label the common ancestor of plants and the origins of multicellular gametangia, vascular tissue, and seeds.

Summary Introduction

To draw: A phylogenetic tree that describes the relationship between the common ancestor of plants and the origins of multicellular gametangia, vascular tissue, and seeds.

Introduction: About 1.2 billion years ago, the microorganisms colonized on the land surface.  The microscopic fossils are the shreds of evidence of life on the Earth. These spore fossils are estimated to be 450 million years old. These spores are different from spores of algae and fungi of the present day in terms of their chemical composition. Cooksonia sporangium is one of the biggest fossils of the larger plants that occurred about 435 million years ago. Then a third clade of seed plants occurred that is defined today as the gymnosperm and the angiosperm.

Explanation of Solution

Pictorial representation: Fig.1 shows the phylogenetic tree drawn to represent the relationship between different groups of land plants.

Fig.1 Phylogenetic tree

Fossils show that the origin of plants occurred about various million years ago. Plants then divided into various groups like the Bryophytes (non-vascular plants), which include liverworts, mosses, and hornworts. Then about 430 million years ago the origin of seedless vascular plants occurred, which includes lycophytes (spike mosses, and quillworts) and monilophytes (fern, horsetail, and whiskfern). Around 360 million years ago various seed plants originate that including angiosperm as well as the gymnosperm, occurred. The Fig.1 shows the phylogenetic relationship between the four groups of the plant.

There are some characteristics that differentiate these four groups of plants are as follows:

• Bryophytes (mosses): The vascular system is absent, the gametophyte stage is dominant, having spores, and have motile sperm
• Pteridophytes (ferns): Vascular system present, dominant sporophyte system, reproduction through spores, and motile sperm is present.
• Gymnosperm (Pines, Spruce, and Gingko): They also have a vascular system with sporophyte as the dominant stage and the reproduction occurs by either heterospory, cones, pollen, eggs, or seeds.
• Angiosperm (monocot and dicots): The vascular system is present and the sporophyte stage is dominant in sporophytes.