recap

27.3 recap

Vascular plants are characterized by a vascular system specialized for the transport of materials from one part of the plant to another. A new type of cell, the tracheid, marked the origin of this group. Later evolutionary events included the appearance of roots, leaves, and heterospory.

learning outcomes

You should be able to:

  • Explain the basic functions of vascular tissues.

  • Describe important structures in vascular plants that probably first evolved in lycophytes.

  • Compare possible selective advantages of heterospory over homospory.

Question 1

How do the vascular tissues xylem and phloem serve the vascular plants?

Xylem conducts water and minerals from the soil to aerial parts of the plant. Xylem also provides structural support. Phloem conducts the products of photosynthesis from sites of production to sites of storage within the plant.

Question 2

Describe the evolution and distribution of different kinds of leaves and roots among the vascular plants.

The earliest rootlike structures were rhizomes—horizontal portions of stem that bear water-absorbing filaments called rhizoids. Rhizomes are retained in some modern plants, such as liverworts and mosses, and are also found in some vascular plants, such as ferns and lycophytes. Ferns and lycophytes, along with other vascular plants, also have true roots, a specialized organ for extracting water and minerals from the soil. Leaves may have originated twice among the vascular plants. The simple microphylls of lycophytes may have arisen from sterile sporangia. In contrast, the larger and more complex megaphylls, or true leaves, may have arisen from the flattening of a portion of a branching stem system that exhibited overtopping growth.

Question 3

Explain the concept of heterospory. How does heterospory provide selective advantages over homospory?

Heterospory refers to the presence of two distinct types of spores, one that develops into a female gametophyte and the other into a male gametophyte. One advantage of heterospory is that it promotes outcrossing among individuals. The fact that heterospory has evolved multiple times from homospory provides evidence that the heterosporous condition is advantageous.