25.1: Introduction

Plants (Kingdom Plantae) are a monophyletic group of eukaryotes that are multicelluar, photosynthetic, and living on land. The most recent evidence suggests that early land plants were most similar to multicellular green algaes (specifically charophytes). As they moved onto land, plants faced a different environment - the ocean environment differs from the land environment in features such as temperature, water availability, sunlight (and UV radiation), buoyancy, and more. With this new environment came challenges that resulted in new selective pressures and different adaptations in those that survived.

Major adaptations of land plants that also define the kingdom include

• Similar life cycles (including alternation of generations)
• Multicellular embryos
• Specialized multicellular structures for protection of gametes, spores, and/or seeds

In this lab you will investigate the diversity of plants on Earth, with a focus on two major innovations of land plants: vascular tissue and seeds. We will group plants into three major categories

1. Nonvascular plants (also called bryophytes). Examples: Mosses, liverworts, hornworts.
2. Vascular seedless plants. Examples: Club mosses, ferns, horsetails.
3. Vascular seed plants. Examples: Conifers, ginkgo, flowering plants

Attribution: Darcy Ernst (Evergreen Valley College)

Nonvascular plants (Bryophytes)

Bryophytes arose in a period of Earth’s history before soils had formed. The terrestrial surface was rocky and consisted primarily of crusts (microbial mats) composed of assemblages of prokaryotes. The exposure to sunlight would have been intense relative to the buffer provided by water. In addition, being surrounded by water would provide regulation of surrounding temperature and structural support. As green algae began to colonize the terrestrial surface, at least one of these lineages accumulated adaptations that were favorable to living on land--a waxy cuticle to prevent water loss, desiccation-resistant dispersal propagules called spores, and retention and feeding of the developing zygote. This lineage of green algae evolved into the ancestor of the bryophytes. This evolutionary group includes liverworts, mosses, and hornworts. These plants do not have true roots to absorb water, nor do they have vascular tissue to transport that water to other regions of the plant. Because of this, bryophytes tend to grow prostrate (close to the surface they are growing on) and stay quite small. They also tend to grow in moist areas where there is access to water and are reliant on water for the dispersal of gametes and fertilization. There are approximately 23,000 known extant species.

The evolutionary relationships between bryophyte lineages are currently unresolved.* Members of this group have the following characteristics:

• Morphology: Multicellular, can be leafy or thalloid. Complex tissues, including an exterior protective layer. Root-like structures called rhizoids provide anchorage.
• Cell wall composition: Cellulose
• Chloroplasts: 2 membranes, pigments are chlorophyll a, chlorophyll b, and carotenoids
• Storage carbohydrate: Starch
• Life cycle: Alternation of generations. Gametophyte dominant: sporophytes grow from and are nourished by the female gametophyte.
• Ecology: Terrestrial, gametes are dispersed in water

*Note: As of 2019, much is unresolved on the early lineages of plants and who was first on land. Recent genetic analyses interpret bryophytes as being monophyletic, all deriving from a common ancestor that branched from the main line of plants. Read this open-access paper for further information: https://doi.org/10.1073/pnas.1323926111

Contributors and Attributions

•  Maria Morrow (College of the Redwoods)

As bryophytes began to colonize the terrestrial surface, they produced organic acids during metabolism that aided in the breakdown of the rocky substrate. When they died, their organic matter mixed with the weathered rock, forming the Earth’s earliest soils. Formerly abundant to the first photosynthesizers to become terrestrial, access to sunlight became competitive as bryophytes expanded. This led to selection for individuals that could lift themselves higher and transport water throughout their tissues. Eventually, this selection resulted in the evolution of vascular tissue -- pipes that could bring water up from the ground so that parts of the plant could be raised upward, and those parts raised upward could transport their photosynthates down to the lower parts of the plant. The cells in the xylem (water-transporting vascular tissue) contained lignin, the tough, decay-resistant compound that wood is made out of. This rigid molecule in the vascular tissue allowed for structural support, allowing plants to grow taller -- some over 100 feet! The vascular system also allowed for the specialization of organs: roots for water absorption, leaves for photosynthesis, and stems for structural support.

Seedless vascular plants (SVPs) also began to rely more on the sporophyte stage. The sporophyte became the larger, nutritionally independent stage of the life cycle. Branching sporophytes offered more sites for meiosis to occur, resulting in increased opportunities for variation, which could be interpreted as more options in an increasingly competitive environment. There are approximately 20,000 known extant species, most of which are ferns.

SVPs are considered to be a paraphyletic group of organisms, forming two distinct lineages: Ferns and Lycophytes. These two lineages share the following characteristics:

• Morphology: Sporophytes develop complex tissues, including lignified vascular tissue, true roots, stems, and leaves. Sporophytes are branched, producing many sporangia. Gametophytes are reduced and thalloid. In some groups, the gametophyte is subterranean and parasitizes mycorrhizal fungi for sugars.
• Life cycle: Alternation of generations. Sporophyte dominant: sporophytes still grow from the gametophyte, but are now photosynthetic and the larger, longer-lived phase of the life cycle.
• Ecology: Gametes are still dispersed in water, so moisture is still required for fertilization.

Selection Pressures and Drivers

1. Competition for sunlight. To get access to sunlight, SVPs needed to grow taller than bryophytes. However, this presents a problem of distributing water around the plant body to prevent drying out. Seedless vascular plants solved this problem with the adaptation of lignified vascular tissue. The lignin in the secondary walls of sclerenchyma cells allowed SVPs the structural support to grow taller.
2. The initial forming of soils. Before the bryophytes, terrestrial surfaces were primarily rocky. The first land plants would have contributed to the chemical weathering of these rocks by producing acids during metabolism. After death, these plants add their organic matter to these weathering rocks, beginning to form Earth’s early soils. Fungi were likely involved in this process, as there are genes in the bryophytes for mycorrhizal relationships.

Gymnosperms

Toward the end of the carboniferous period, major changes in the climate occurred. The current day European and North American continents slammed together, forming the Appalachian mountains (which were taller, at that time, than the present-day Himalayas). Fossil and geologic records show a tendency toward a drier climate, with evidence of glaciation and lowered sea levels. Inland seas were increasingly diverted into distinct river channels as woody debris channelled the movement of waterways. In short, the terrestrial surface began to dry out and there was much more of it. The ancestors of birds, reptiles and mammals were adapting eggs that could survive outside of the water -- plants were working toward a similar strategy. Around this time, a group of animals likely took flight for the first time -- the insects! The presence of flying insects allowed for another option for distributing pollen, as well as a large source of potential herbivory.

The plants that would become the gymnosperms evolved xerophytic leaves (see lab 7) to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem (wood) each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones.

The fossil record shows gymnosperms diversifying in a dry period called the Permian that followed the swampy Carboniferous period. Extant groups of gymnosperms include conifers, cycads (somewhat similar in appearance to palms), gnetophytes, and a single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1,000 species of gymnosperms alive today, about 600 of these are conifers, 58 of which can be found in California. In fact, some of the oldest (bristlecone pine), tallest (coast redwood), and most massive (giant sequoia) organisms on the planet are conifers and all are native to California.

See this open-access paper for recent genetic work on the evolutionary relationships between gymnosperms: http://dx.doi.org/10.1098/rspb.2018.1012

Selection Pressures and Drivers

1. Competition for sunlight. Seedless vascular plants were able to reach heights up to 100 feet tall. In the lineage leading to the gymnosperms and angiosperms, some plants developed the ability to grow wider as they grew taller. This secondary growth allowed for increased stability and, eventually, to reach heights over 300 feet.
2. Drought. Dry conditions would have selected for plants with thicker cuticles, leaves with less surface area to evaporate from, and propagules that could disperse without water and survive through dry periods to germinate when water was available.
3. Herbivory. In addition to leaves that could resist drought, the presence of insects would have driven selection for plants that could defend against herbivory. The thick cuticle and tough texture of xerophytic leaves made them difficult to eat, while resin canals in both leaves and stems provided another line of defense.

Angiosperms

The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions or other circumstances. However, there is relatively new fossil evidence of flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Angiosperms represent a single origin of related organisms, the phylum Anthophyta, that experienced an exceptional radiation in species. As of 2019, there are approximately 370,000 known extant species. Most of the plants that you see, eat, and otherwise interact with in your daily life are likely to be in this group.

Angiosperms can be distinguished from other plant groups by the production of flowers. These collections of modified leaves allowed angiosperms to attract pollinators and increase the chances of successful fertilization. Over time, angiosperms evolved different flower morphologies, smells, and colors that corresponded to their particular pollinators. These sets of characteristics, called pollination syndromes, allow scientists to predict the pollinators for different plants.

In the xylem, this group of plants evolved large diameter conducting cells for rapid water uptake called vessel elements, though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements with their associated companion cells, increasingly specialized for transportation of photosynthates.

Selection Pressures and Drivers

1. Competition for space. Present day gymnosperms include the tallest, most massive, and some of the oldest organisms on the planet. With this in mind, you can imagine that they would be difficult to compete with. Angiosperms needed to evolve more efficient methods of transporting water and photosynthates, fertilization, and survival of offspring.
2. Insects, birds and mammals. The primary response to insects that we see in gymnosperms is prevention of herbivory. While herbivory is still a driver of selection for angiosperms, insects also served as a more efficient method of pollen delivery. Insects and birds could be lured in with sugary nectar or scents and colors that mimicked other resources, then dusted with pollen as they investigated. If the lures were specialized enough, they would continue seeking the same resource, leading them to another plant of the same species. These scents, colors, and nectar resources were produced by structures that also produced pollen and ovules -- the flower. The diversity of animals present on Earth when angiosperms evolved would have resulted in increased seed predation. Fruits allowed for the dual purpose of protecting seeds and co-opting animals as dispersal agents, whether by ingestion or attachment. Some fruits evolved production of sugary tissues and bright colors to attract animals, while others evolved hairs or spines to latch onto their bodies.

The term angiosperm means “seed vessel” and refers to the production of fruits. Every flower becomes a fruit, though these fruits might not always fit with our cultural perception of what it means to be a fruit. Once pollinated, the fertilized seeds are encased in a protective ovary whose structure can be specialized for different methods of dispersal, such as animal ingestion, animal attachment, flotation, or wind dispersal. This protective ovary and the encased seed or seeds are more commonly called a fruit. Inside the developing seeds, angiosperms provide an additional food source to the developing zygote, the endosperm. The endosperm is produced by a process called double fertilization where one sperm fertilizes the egg and another fertilizes a pair of haploid nuclei, which makes the endosperm triploid (3n).