Lecture 25: Seed Plants - Angiosperms Part II, Fruits and Flowers

Fruits

Fruits protect the seed and embryo and facilitate the dispersal of the new sporophyte. Fruit morphologies vary widely depending on the dispersal method. Similar to pollination, fruit dispersal plays a significant role in the evolutionary radiation of angiosperms.

Fruit Development

Fruit development, generally involves the ovary tissue.

Fruit Morphology

True fruit consists only of ovary tissue. The ovary wall (pericarp) differentiates into three tissue layers: the endocarp, mesocarp, and exocarp. The pericarp can be either fleshy or dry. If dry, it can either break open (dehisce) or stay intact.

A simple fruit develops from a single ovary within one flower.

Drupe

A drupe is a type of fruit where the endocarp is tough and sclerified.

Berry

In a berry, the pericarp is fleshy throughout.

Aggregate fruit

"Berries" like raspberries and blackberries are aggregate fruits, which are made from fused ovaries on one flower, not a single ovary.

Multiple fruit

Multiple fruits, such as pineapples and jackfruit, develop from many flowers on an inflorescence, forming a fused macro "fruit" structure.

Accessory fruit

Accessory fruits develop from tissue other than the ovary. For example, the fleshy part of a strawberry is derived from the flower's receptacle.

Achene

Strawberry's true fruit is an indehiscent dry fruit called an achene, where the seed nearly fills up the pericarp.

Caryopsis

Grasses, such as corn, exhibit a caryopsis, where the pericarp is fused to the seed coat. These are usually wind dispersed.

Samara and Cypsela

A maple samara is a winged achene adapted for wind dispersal. Dandelions produce a different winged achene called a cypsela, while the cotton dry fruit (capsule) grows long trichomes for wind dispersal.

Some fruits are adapted for water dispersal, such as coconuts.

Parthenocarpy

Fruits can develop without seeds through a process called parthenocarpy. An example is the 3N sterile offspring of a cross between a 2N banana and a 4N banana.

Polyploidy

Many cultivated crops are polyploid. Sterile plants reproduce through methods such as vegetative propagation.

Human-Plant Coevolution

Six species of flowering plants make up 80% of the calories consumed by humans. Plant domestication represents a significant achievement in selective breeding and genetic engineering.
Flowering plants produce tens of thousands of secondary metabolites with human applications. The loss of forests decreases biodiversity and contributes to climate change by reducing CO_2 absorption. Plant diversity takes a very long time to recover and is not a renewable resource.

Angiosperms

Flowering plants, or angiosperms, dominate Earth today, with over 250,000 to 300,000 plant species. They possess many derived features and complex lifestyles, including secondary metabolites and relationships with pollinators and seed-dispersing animals. Their greater plasticity allows them to thrive in diverse environments.

Evolution is not a linear progression. Populations encounter isolation barriers and diversify until some members can no longer reproduce with others, leading to the formation of new species. Ferns have also diversified into numerous habitats, and there is evidence that conifers arose within gymnosperms after angiosperms evolved.

Trends in Plant Evolution

Increasing Prominence of Sporophyte, Decreasing Prominence of Gametophyte

With Haplontic life cycles, there is no multicellular diploid stage.

Increased Protection of Embryo

Ovary surrounds the ovule, and the ovule protects the embryo. There is also embryo retention
The ovary is "sunken" in many derived forms, within the hypanthium.

Decreasing Reliance on Water

Increasing Complexity of Growth/Structures

It is important to understand these trends and be able to provide examples that illustrate each one.