Evolutionary Trends & Diversification in Plants (Lecture 6)
Evolutionary Trends in Plants & Diversification of Angiosperms
- Evolutionary trend in plants
- Characteristics of angiosperm
- Types of Flower
- Types of Inflorescence
- Mode of pollination
- Flower types affecting self & cross pollination
Key “Moments” in Plant Evolution
- The Transition to Land
- Development of Vascular Systems
- Evolution of Heterospory
- Evolution of the Seed
- Diversification of the Angiosperms
Evolutionary trend in plants
The major groups of plants are separated by three distinct features;
- Vascular tissue
Rise of Vascular plants
- The first land plants lacked vascular tissues, so they could not transport water, sugars or minerals around the plant.
- Lack of vascular tissue, of course, limited the size of plants.
- Once the first plants moved onto land, selection quickly led to the development of specialized roots and shoots.
- Roots and shoots required the development of a vascular system to move water and other essentials around the plant and by about 400mya early vascular plants had begun to diversify.
Transition from homospory to heterospory
- Homospory means spores are the same size and heterospory that microspores (male) and megaspores (female) differ in size.
- Microspores develop into male gametophytes and megaspores into female gametophytes.
- Mosses and most ferns are homosporous. Conifers and flowering plants are heterosporous.
- Homosporous plants produce spores that develop into bisexual gametophytes that produce both sperm and eggs.
- For successful fertilization, homosporous plants need water in the form of rainfall when gametes are mature.
- Some homosporous plants evolved heterospory.
- With heterospory in which the female gametophyte is enclosed and protected and there is no need for water to ensure fertilization.
- Heterospory led to the evolution of seeds.
Evolution of the seed
- In mosses, the life cycle is dominated by the gametophyte generation.
- In ferns, the sporophyte generation is dominant and the gametophyte is reduced, but still visible to the naked eye.
- In seed plants, the gametophyte generation is so reduced that in most cases it is microscopic.
- Alternation of generations
dominant haploid plant
- bryophytes – mosses
dominant diploid plant
- Pteridophytes – ferns
- Gymnosperm – conifers
- Angiosperm – flowering plants
- Evolutionary advantage? reduction of gametophyte protects delicate egg & embryo in protective sporophyte
- The angiosperms have been immensely successful.
- There are now about 235,000 species in comparison to just over 700 gymnosperms.
Gametophytes of seed plants
Characteristics of Angiosperms
- The key adaptations in the evolution of angiosperms
Are flowers and fruits
Modified shoot with 4 rings of modified leaves
Parts of flower
Peduncle: The stalk of a flower.
Receptacle: The part of a flower stalk where the parts of the flower are attached.
Sepal: The outer parts of the flower (often green and leaf-like) that enclose a developing bud and flower.
Petal: The parts of a flower that are brightly colored and attract pollinators
Male Parts of flower
- Stamens = male reproductive organs
- stalks (filament) & terminal anthers
- pollen sacs produce pollen
- pollen grain = gametophyte
- sperm-producing structure
Female Parts of flower
- Female carpels = female reproductive organs
- Stigma = top of carpel—–Sticky-Collects pollen
- slender neck = style
- ovary at the base
- within the ovary are 1 or more ovules
- within ovules are embryo sacs
- female gametophyte = embryo sac
- egg-producing structure
- Complete Flower
- Incomplete Flower
- Perfect Flower
- Imperfect Flower
Complete vs. Incomplete Flower Types
- There are different types of flower according to the presence of the different parts in the same floral structure.
- A flower is referred to as complete if all four floral organs (sepal, petal, stamen and pistil) are present in the same flower structure. A commonly illustrated complete flower is that of the China rose (Hibiscus rosa-sinensis).
- An incomplete flower lacks any one or more of these parts. Grass flowers are mainly wind-pollinated and are incomplete, lacking both sepals and petals. Relying on wind to accomplish pollination, there is no need for these organs to attract pollinators.
Perfect vs. Imperfect Flower Types
- Flowers that contain both sexual flower parts (stamen and pistil) are called perfect or bisexual
- while those that contain either stamen or pistil only are called imperfect or unisexual flowers, regardless of whether they lack sepals or petals.
- The separation of the male and female sexual organs increases the possibility of outcrossing or cross pollination.
- Imperfect flowers are found in corn (Zea mays), squash (Cucurbita maxima), bitter gourd (Momordica charantia), watermelon (Citrullis lunatus), and coconut (Cocos nucifera).