SURVIVAL ON LAND POSES SOME SERIOUS CHALLENGES to ter-restrial plants, foremost of which is the need to acquire and retain water. In response to these environmental pressures, plants evolved roots and leaves. Roots anchor the plant and absorb water and nutrients; leaves absorb light and exchange gases. As plants increased in size, the roots and leaves became increasingly separated from each other in space.
Thus, systems evolved for long-distance transport that allowed the shoot and the root to efficiently exchange products of absorption and assimilation. You will recall from Chapters 4 and 6 that the xylem is the tissue that transports water and minerals from the root system to the aerial portions of the plant. The phloem is the tissue that translocates the products of photosynthesis from mature leaves to areas of growth and storage, including the roots. As we will see, the phloem also redistributes water
and various compounds throughout the plant body. These compounds, some of which initially arrive in the mature leaves via the xylem, can be either transferred out of the leaves without modification or metabolized before redistribution.
The discussion that follows emphasizes translocation in the phloem of angiosperms because most of the research has been conducted on that group of plants. Gymnosperms will be compared briefly to angiosperms in terms of the anatomy of their conducting cells and possible differences in their mechanism of translocation. First we will examine some aspects of translocation in the phloem that have been researched extensively and are thought to be well understood. These include the pathway and pat-
terns of translocation, materials translocated in the phloem, and rates of movement. In the second part of the chapter we will explore aspects of translocation in the phloem that need further investigation. Some of these areas, such as phloem loading and unloading and the allocation and partitioning of photosynthetic products, are being studied intensively at present.