Lecture 11: Mosses and Land - Biology Lecture Notes

(I started this lecture by finishing lecture 10 on Protists) LECTURE 11: Mosses & Land We are now going to work up the phylogeny of life and move from the Protists to Land Plants. The Origin of Land Plants Plants evolved on the order of 475 million years ago and are, in some ways, fairly recent....

(I started this lecture by finishing lecture 10 on Protists) LECTURE 11: Mosses & Land We are now going to work up the phylogeny of life and move from the Protists to Land Plants. The Origin of Land Plants Plants evolved on the order of 475 million years ago and are, in some ways, fairly recent. However, they are of the utmost importance to life as we know it, because plants were the first group of complex multicellular eukaryotes to successfully invade land. As I mentioned previously, plants probably evolved from green algae. For billions of years, biodiversity was restricted to the ocean. In fact, most major animal phyla evolved before there were plants. Development of terrestrial animal life was completely dependent on land plants, because as autotrophs they served as a primary food source and also as a source of shelter. This leads to a definition of land plants as multicellular photosynthetic autotrophic eukaryotes that can survive and reproduce on land The move to land was not trivial - it required at least two important innovations: 1. Sexual reproduction on land. For plants to succeed on land, they had not only to survive on land but also to reproduce on land. Without reproduction on land, they could never have escaped a semi-aquatic existence. Recall that green algae required water for gametes and spores to swim. As we will see, plants have become less and less dependent on water for reproduction over time. A key feature of reproduction on land was the evolution of the embryophyte, which literally means “embyro bearing plant”. All land plants are embryophytes and have life-cycles that include the alternation of generations. I’ll go into the details of embyrophytes in the last half of this lecture, but in general terms embryophytes are organisms that have a structure that nourishes and protects the developing diploid embryo. 2. The ability to survive on land and in contact with air. 1  Plants no longer had a homogeneous environment, in which all parts of the plant have contact with nutrients, and they no longer grew in a medium which provided structural support. One of the major innovations was the formation of a waxy cuticle layer that helps a plant retain moisture, along with a series of pores for gas exchange called stomata. Vascular tissue was formed in response to a heterogeneous environment. Vascular tissue transports water and nutrients throughout the body of a plant. In addition to being the transport system for nutrients and water, vascular tissue has two additional functions. First, vascular tissue acts to support the plant body. Second, vascular tissue is part of a system that helps avoid dessication (drying out). We’ll talk extensively about vascular tissue in the next lecture. Four major groups and four major evolutionary innovations *** We only got partway through this section in lecture! We’ll finish this lecture on Wednesday!*** There are four main groups of land plants. Before the evolutionary divergence of each group, there was at least one major evolutionary innovation. Figure 28.5 & 28.9 1. The first group of plants, and the one that may best represent the earliest land plants, is the non-vascular plants. This is a diverse group of plants, but we will focus mainly on what these plants can tell us about the first major innovation - i.e., the origin of the embryophyte. 2. After the divergence of nonvascular land plants, there was a second major innovation: the origin of vascular tissue. Next lecture we’ll focus on vascular tissue and seedless vascular plants, which includes ferns. (Note that some nonvascular plants contain rudimentary vascular tissue, but it is probably not evolutionarily homologous to vascular tissue in ferns, gymnosperms and angiosperms.) 3. After the divergence of ferns, there was another important innovation: the evolution of the seed. We’ll discuss the evolution of seeds in conjunction with the gymnosperms, a term that literally means “naked seeds”. Pine trees are an example of gymnosperms. 4. The fourth innovation was the evolution of the flower, which led to the angiosperms or ‘flowering plants’. The angiosperms have been the most successful in terms of the number and distribution of species. Angiosperms account for most of the plants we see today. 2  Group Number of Species Nonvascular 1,150 Plants Seedless 11,000 Vascular Plants Gymnosperms 900 Angiosperms 250,000 The groups and their key innovations are summarized below. Group Embryophyte Vascular Seed Flower Tissue Nonvascular Yes No/yes No No Plants Seedless Yes Yes No No Vascular Plants Gymnosperms Yes Yes Yes No Angiosperms Yes Yes Yes Yes In addition to the development of the embryophyte (or ‘embryo bearing plant’), I should reiterate that there are two additional innovations that probably evolved in the common ancestor of the four major group.These two innovations help plants survive in a non- aqueous environment and are common to all land plants. They are: 1. A waxy cuticle layer that helps retain moisture in tissues. 2. The presence of a gas exchange pores known as stomata are in some seedless non-vascular plants. These pores are usually found on leaves. They are important for many reasons, but most importantly they allow the exchange of gases (CO2 and O2) with the environment. (If the plant was completely covered with an impermeable cuticle then the plant couldn’t survive.) 3. [Your book also mentions a group of chemical called flavonoids, which may act as a sort of a ‘sunscreen’ to provide some protection against UV rays. I’m happy to mention them, but they may not be as important as the cuticle layer and stomata.] Non-vascular plants Non-vascular plants are embryophytes that have a waxy cuticle layer and some have stomata. However, non-vascular plants lack true vascular tissues, which makes transport of water and nutrients from the soil to the leaves difficult. As a result, non-vascular plants are limited to very moist environments, and they don’t grow tall. Their short stature ensures that most parts of the plant are near moisture. 3 As I mentioned previously, non-vascular plants are embryophytes. For most of the rest of this lecture, I’d like to consider the life-cycle of a moss and, in the meantime, try to provide an understanding of embryophyte reproduction. Figure 28.13 Note the following features of the moss life cycle: 1. It has alternation of generations. 2. The gametophyte is dominant. As we will see over the next few lectures, sporophytes are the dominant form of ferns, gymnosperms and angiosperms. 3. Male and female gametophytes are independent; there are separate male and female producing spores. 4. Sperm develop in the antheridium, which is the male reproductive organ. Motile sperm are released from the antheridium and swim to the egg, which is located in the archegonium. 5. The egg develops in the archegonium. The archegonium is a structure of sterile (non-reproducing) cells that shelters the egg. The egg is fertilized in the archegonium when the sperm swim into it. Diagram of archegonium. 6. The zygote (fertilized egg) develops into an embryo in the archegonium. Altogether, the archegonium helps protect the egg, the zygote and the embryo from dessication (drying out). The archegonium is the defining feature of embryophytes and an important adaptation for reproduction on land! 7. In mosses, the sporophyte grows out of the archegonium and is physically attached. The sporophyte is dependent upon the gametophyte for water and nutrients, and the sporophyte is usually short-lived. 8. Important! Fertilization still requires water, because the motile sperm need water to reach the archegonium and fertilize the egg. Mosses, although living on land, are still quite dependent on water for fertilization! In short, two things limit the distribution and size of mosses. First, they still require water for fertilization, so they can’t inhabit dry environments. Second, they don’t have a good vascular system. This means that most of the plant needs to be near water to ensure that all the cells get water. This limits the size of mosses. Typically they are very short (a few centimeters in height). 4