No one really knows what a tree is - Max G. Levy

Key Concepts:

• True Trees: Tall, woody plants with leaves and a single load-bearing trunk, exhibiting both primary and secondary growth.
• Primary Growth: Initial upward growth of a soft stem.
• Secondary Growth: Bulking out of the stem into a woody trunk.
• Gymnosperms: Seed plants that are all woody, some of which are trees.
• Angiosperms: Flowering plants, some woody and some trees.
• Shrubs: Woody plants with multiple stems.
• Herbs: Plants with non-woody stems.
• Dicots: Angiosperms with two seed leaves, capable of woody secondary growth.
• Monocots: Angiosperms with one seed leaf, lacking woody secondary growth.
• Cauliflory: The trait of bearing flowers and fruit directly on the trunk and branches.
• Pseudostem: A false stem formed by tightly overlapping leaf bases, as in bananas.
• Arborescent: Having the characteristics of a tree.

Defining a True Tree

The video begins by posing the question of what defines a tree, noting that while many plants may appear to be trees, only some meet the botanical criteria. True trees are generally defined as tall, woody plants possessing leaves and a single, load-bearing trunk. The development of a tree involves two key phases: primary growth, where a soft stem grows upwards, and secondary growth, where the stem thickens into a woody trunk. Trees continue to grow taller from their crown and wider as new rings expand their trunks annually.

Seed Plants: Gymnosperms and Angiosperms

The video explains that trees are found exclusively among seed plants, which are divided into two groups: gymnosperms and angiosperms. All gymnosperms are woody plants, and some are trees. Angiosperms, or flowering plants, include both woody and non-woody species, with only some being classified as trees.

Plant Forms: Trees, Shrubs, and Herbs

Plants exhibit a variety of forms, including trees, shrubs, and herbs. Shrubs, such as lavender (an angiosperm) and ephedra (a gymnosperm), grow as clusters of larger, woody stems. Herbs, like mint (an angiosperm), have herbaceous, non-woody stems. The video highlights that plant forms are evolutionarily intertwined, and trees do not constitute a single, closely related group.

Evolutionary Paths to Tree Status

Many plants have followed distinct evolutionary paths to become trees. For example, apple trees are more closely related to rose shrubs and strawberry plants than to avocados and guava trees. Over millions of years, some plant lineages have become less tree-like, while others have become increasingly arborescent.

Monocots vs. Dicots: The Key to Woodiness

The video delves into the differences between monocots and dicots, two groups within angiosperms. While all plants exhibit primary growth, and all gymnosperms display secondary growth, angiosperms are divided into monocots and dicots. Dicots sprout two seed leaves, while monocots sprout only one. Monocots do not undergo woody secondary growth, meaning none of them are trees. Some dicots, however, do exhibit woody secondary growth and can become trees.

The reason dicots can grow wood and become trees while monocots cannot is rooted in evolutionary history. The earliest angiosperms were likely woody, similar to their non-flowering relatives, the gymnosperms. Dicots appear to have retained the genetic propensity for woodiness, even though it has been switched off in some lineages, while monocots seem to have lost it entirely.

Examples of True and False Trees

The video revisits the initial six plants, identifying the true trees as the Brazilian grape tree, giant baobabs, and bristlecone pines. The Brazilian grape tree exhibits cauliflory, where fruit grows directly from the trunk and branches. Giant baobabs have bulbous trunks that store water. Bristlecone pines are slow-growing and sturdy, adapted to cold, windy environments.

The remaining three plants—bananas, Joshua trees, and palms—are not true trees because they are monocots. Bananas have pseudostems made of hardened, overlapping leaves. Joshua trees are succulents. Palms are closely related to grasses.

The Blurry Line Between Shrub and Tree

Among angiosperm dicots that produce wood, the distinction between shrub and tree can be ambiguous. Characteristics such as height and trunk diameter are often used to differentiate between them, but these metrics can change over time. A plant like juniper may transition from shrub to tree within its lifetime.

Conclusion

Plant evolution is complex, and the definition of a tree is not always straightforward. Even plants that do not technically qualify as trees possess tree-like characteristics and deserve recognition. The video concludes by acknowledging the diversity and adaptability of plants, regardless of their classification.