Further Reading

After all, I guess it doesn't matter whether you look down [through a microscope] or up [through a telescope] — as long as you look.

John Steinbeck, Sweet Thursday it is the presence of green vegetation on the surface of the Earth that makes it a pleasant and interesting place to live. Frequently, we take this green mantle for granted, forgetting that for most of earth history the landscape was barren. Cyanobacteria, algae, and algal-like organisms must have lived in terrestrial habitats before true land plants evolved, but surely they did not have the same effect on the appearance of the Earth as true land plants do. We now know that the early land flora included both vascular and thalloid forms (Chapter 6), as does the terrestrial flora today. This chapter will be restricted, however, to a discussion of land plants with vascular tissues. In some systems of classification, vascular plants are placed in a formal division, the Tracheophyta, and molecular data support monophyly for the vascular plants (Nickrent et al., 2000). In this book, the groups of vascular plants are elevated to phylum level (see Chapter 1), in part reflecting the fossil record of several of these groups, especially the early evolution of vascular plants (Chapter 8).

One of the perplexing problems in the history of plant life has been the long interval between the appearance of green, photosynthetic organisms and the evolution of vascular land plants. There is compelling evidence that autotrophic organisms existed at least 2.5 billion years ago (see Chapter 2). There is fossil evidence that plants with conducting systems which functioned like modern vascular plants existed in the Late Silurian, a little more than 400 million years ago. There is also fossil evidence that land organisms were around earlier, perhaps during the Ordovician, based on the spore and microfossil record. As you have seen in Chapter 6, some of these, no doubt, represented relatively short-lived attempts to colonize the land surface. We now know that some of the plants that have traditionally been regarded as

figure 7.1 Cross section of Aglaophyton major stem showing conducting strand (Devonian). Bar = 500pm. (Courtesy H. Kerp.)

the earliest vascular land plants, for example, Cooksonia and Aglaophyton (FIG. 7.1) , have conducting elements that are different from those of true vascular plants and which may have had a different evolutionary origin (see Chapter 8).

Since vascular plants contain rigid, resistant conducting tissues and a waterproofing cuticle, they are much more frequently found fossilized than algae, fungi, and bryophytes. This chapter is a brief review of vascular plant morphology and anatomy for those who have not studied these subjects previously, including a brief outline of the principal cell types, tissue systems, and structures found in vascular plants. It is a general introduction to the subject and does not take into account the many exceptions to the definitions and principles listed in the following sections. For more detailed information on morphology or anatomy, we have listed some additional sources at the end of this chapter.

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