Plants II: Vascular Plant Structure

This lab is about how vascular plants function and how they grow. As described in Plants I, terrestrial plants face difficult environmental challenges not encountered by aquatic algae. Vascular tissue structure and function is at the heart of the ways that plants adapt to these challenges, which is why most terrestrial plant communities are dominated by vascular plants. In this lab you'll examine the connections between the structural complexity and the mechanisms of growth of vascular plants.

The mechanisms of transport in vascular tissue will be covered in lecture in the section on plant transport, and in Campbell Biology, Chapter 36.

Objectives

• Identify examples of dermal, ground, and vascular tissues in all the specimens in lab.
• Explain what meristematic growth is, and why it is an essential feature of land plants but not algae.
• Compare & contrast apical and lateral meristems, and recognize which plant tissues were produced by which type of meristem.

Other pages for this lab contain additional objectives.

Why plants need vascular tissue

One basic problem facing land plants is that the water and nutrients are underground, but the sunlight is above ground.

Each cell of a vascular plant is no more complex than an algal (protist) cell, and yet plant bodies are much more complex than protists. This complexity is at the level of tissues and organs. Vascular plants are more complex in their body organization than non-vascular plants, and are much more complex than photosynthetic protists such as seaweeds.

Tissues

A tissue is a group of cells with similar appearance and function. Plants have three major tissue systems:

• Dermal tissues cover the entire outside of the plant – roots, stems, and leaves. Dermal tissues include the epidermis of leaves and green stems and the outer layer of bark in woody plants.
• Vascular tissues transport water and other molecules throughout the plant body. Vascular tissues include xylem and phloem, which you’ll study in detail later.
• Ground tissues make up the rest of the plant, including the mesophyll cells responsible for photosynthesis inside the leaves and pith in the inner part of stems.

Organs

An organ is a structure that carries out a particular function and contains several kinds of tissues. For example, a leaf is an organ; leaves have several kinds of tissues. Vascular plants have three basic kinds of vegetative (non-reproductive) organs: leaves, stems, and roots. In this lab, you’ll look at some of the wide variety that exists within each of these categories in terms of structure and function.

Tissue & organ complexity creates a growth problem

For a simple organism with no tissue or organ differentiation, growth can occur anywhere; there is no need for a specific pattern. This is, in fact, how growth occurs for many green algae. However, for vascular plants, all the required tissues must be produced at the right place and time and in the right relationships. Also, the woody structure of large plants allows growth to occur only in particular parts of the plant.

Meristematic growth is an essential part of solving the growth problem. Land plants (embryophytes) grow only at meristems, which are groups of cells that specialize in cell proliferation. The meristems occur in specific places on the plant to allow organized growth. You may remember from the last lab that the large, long-lived part of a vascular plant life cycle is the diploid sporophyte. A sporophyte begins as a zygote (a fertilized egg). The development of the adult plant body from the zygote requires two processes: cell proliferation to make new cells and differentiation to give those cells their proper identity. (Later, you’ll see that both these processes also occur in animal development, but in a very different way.) All the cell proliferation occurs at meristems. The newly produced cells are undifferentiated, and later differentiate into specific types. There are meristems in the roots and in the shoot (the part above ground). Apical meristems at the tips of roots and stems make the plant grow longer and produce leaves. This is called primary growth; it produces all the tissue types in a plant. Woody plants get thicker by adding rings of secondary growth at lateral meristems. You’ll see how this works in detail as you look at stems.

Long-lived plants have indeterminate growth. This means that they can continue growing throughout their lives, can change their growth pattern in response to their environment, and can replace parts that get damaged. Animal growth is normally more determinate; for example, once you've grown two arms you don't keep growing more. Individual plant organs such as leaves also show determinate growth. The problem with indeterminate growth is that a plant (a tree, for example) must be structured in a way that functions when it is small and continues to function as more cells are added over the years.

What to do in lab

Go to the pages on roots, stems, and leaves. Look at the specimens listed there and observe the features described there and in the appropriate diagrams of Campbell Biology, Chapter 35. Once you feel you've got an understanding of the concepts, test yourself with the plant lab review page. You don't need to turn anything in today; focus on learning the material for the lab exam.

References & further reading:

Campbell Biology, Chapter 35: Plant Structure, Growth, and Development. This chapter had detailed diagrams of the types of structures you'll see in this lab.

Campbell Biology, Chapter 36: Resource Acquisition and Transport in Vascular Plants. This chapter details the mechanisms of transport by vascular tissue, a topic that will be covered in lecture.