Vascular Plants

Palm trees lining the road in a sunny place, ferns coating the ground in a thick, damp forest, cacti dotting the landscape in the arid desert: what do these plants all have in common? They are all part of a large group of plants known as the tracheophytes, or vascular plants.

Vascular Plants: Definition

What makes a vascular plant? Vascular plants share a common trait that separates them from other plants, a vascular system. This vascular system is composed of xylem and phloem tissue, which help transport nutrients, carbohydrates (sugars), and water throughout the plant.

Two other characteristics that define vascular plants are:

1. Their roots, leaves, and stems are “true” because they have vascular tissue.

2. The sporophyte, or diploid, generation is the dominant generation (the plant’s generation spends most of its life cycle in).

A Successful Adaptation

The vascular plants make up 80% of all plant species. In other words, most plants on earth are vascular plants! What is the advantage of having a vascular system?

Think about it for a second: if you couldn’t move and didn’t have a way to transport water from one part of your body to another, it would be easy to dry out quickly unless in a damp environment. Thus, having a vascular system is beneficial for living on land.

Additionally, non-vascular plants that live on land are often small because, without a way to transport nutrients and water within themselves, the plant cannot grow as large. The evolution of the vascular system in plants allowed vascular plants to grow larger and occupy different niches. Thus, contributing to the variety of sizes we see today, from ferns to giant sequoia trees.

The Vascular System in Plants

Think about what your own vascular system does for you: it transports oxygen, nutrients, and essential chemicals from one part of your body to another. Without it, it would be impossible to carry out daily functions, like breathing and absorbing nutrients. In vascular plants, their vascular system plays a similarly important role.

Plants carry out photosynthesis, which uses carbon dioxide, water, and photons from the sun to make carbohydrates that the plant can use to carry out life processes necessary for survival. Therefore, having a vascular system to transport water from the roots to leaves, where photosynthesis occurs, and to transport sugars produced in the leaves to other locations in the plant is important.

Vascular Tissue in Plants

The vascular tissue in plants is called the xylem and phloem. The primary responsibility of the xylem tissue is to transport water and minerals from the roots to the leaves or other parts of the plant. The phloem is used to transport sugars, which function as food for the plant, to parts that cannot produce their own food.

Vascular tissue provides structural support for the plant and varies in arrangement and complexity depending on the group of plants. Typically, the xylem and phloem are packed together, forming vascular bundles (Fig. 1). The tissues’ arrangement creates tubes that run the length of the plant.

A cross-section of a sunflower stem showing the vascular bundles, xylem, and phloem.

Xylem

The xylem of plants consists of cells that are not alive and fortified with a protein called lignin. Lignin provides structural support for the xylem tissue and the plant, and the cells that contain this protein are known as “lignified”.

Flower-producing plants (angiosperms) have xylem made up of two types of cells: tracheids and vessel elements. Other groups, including the gymnosperms (conifers, etc.) and ferns and their allies, only have tracheids that make up the xylem tissue.

Phloem

Phloem consists of alive elongated cells which are not “lignified” like the xylem cells.

In gymnosperms and the ferns and their relatives, the phloem is made up of sieve cells. In flowering plants (angiosperms), the cells are called sieve tubes and feature some structural differences from the cells of other vascular plants.

How Does the Vascular System Work?

In a vascular plant, the leaves lose water through a process known as transpiration. This is the evaporation of water that occurs when the leaves open small pores between their cells called stomata, which allow carbon dioxide necessary for photosynthesis into the plant. Stomata can be opened and closed to let in gas while reducing water loss; however, some water still evaporates.

This evaporation decreases the water pressure at the point of transpiration, causing water to be absorbed by the roots and pulled upwards through the xylem tissue to the leaves, replacing the water lost. The xylem only flows in one direction, from roots to leaves.

The phloem can move in both directions through the vascular plant, as the sugars and nutrients move from sources (leaves, places where photosynthesis happens) to sinks (roots, places of growth). This process of the sugars moving from souse to sink is known as translocation. The theory behind transport through the phloem is that the influx of sugars causes water (from the xylem) to rush into the phloem, creating pressure and a solution that moves toward the sink. This is known as the pressure-flow hypothesis.

Examples of Vascular Plants

There are several types of vascular plants, including clubmosses, horsetails, ferns, gymnosperms (including conifers), and angiosperms (flowering plants).

Vascular plants are also called tracheophytes, but they are split up into several groups based on their properties. Most notably, the non-seed producing and the seed-producing groups.

• The non-seed producing groups include ferns, clubmosses, and horsetails. Instead of seeds, members of this group have an alternation of generations or the switch between diploid and haploid plant generations. The sporophyte generation is the dominant generation, like in other vascular plants.

• Seed-producing plants are split up into gymnosperms (conifers, etc.) and angiosperms (flower-producing). Gymnosperm seeds are referred to as naked because they are typically exposed on a leaf or a cone structure. However, the angiosperm seeds are covered in an ovary (e.g., a fruit).

The vascular tissue, its components, and its arrangement differ between the three groups of vascular plants: ferns and allies, gymnosperms, and angiosperms (Fig. 2).

Vascular Plants Cross-section of a sunflower, with xylem and phloem StudySmarter

Differences Between Vascular and Non-Vascular Plants

There are a few key differences to remember between vascular and non-vascular plants. The table below summarizes these differences (Table 1).

Table 1: Summary of the differences between vascular and non-vascular plants. StudySmarter Originals, Hailee Gibadlo.