Roots and stems

[Bernard NebelKeymaster]

Roots

Roots, in addition to anchoring the plant in the soil, are responsible for absorbing water and mineral nutrients such as phosphate (providing the phosphorous ( P )  seen in DNA molecules) nitrate (providing the nitrogen (N) seen in proteins) and others. (The C, H, and O portions of such molecules comes from glucose.

The root system is much more ramified than commonly thought. It branches, re-branches, and has side shoots including root hairs, single-cell offshoots near root tips that have the appearance of fuzz. See:

https://www.istockphoto.com/search/2/image?phrase=root+hair

Germinate seeds on moist paper towel to see these firsthand. 

In short, the root is so ramified that it contacts nearly every particle of the soil in which it is growing facilitating the uptake of nutrients if they are present at all. 

Looking at a cross section of a root, e.g., 

https://www.vectorstock.com/royalty-free-vector/diagram-showing-cross-section-a-root-vector-27522361

Skip labels except for xylem and phloem cells. Where did we see these before? In the tiny vines within leaves. Yes, this is the other end of the same “tubes.”  They are continuous all the way from the root tips to stem tips and leaves. Xylem provides for the flow of water with mineral nutrients in solution up to all parts of the plant; phloem provides for the transport of glucose to wherever it is needed for energy (growing stem tips, root tips, developing flowers and fruits, etc.) or to be stored.

Stems

Ask kids what they should expect to find as they examine a cross section of a stem. (They can do this firsthand. Use a very sharp scalpel/razor blade to a cut a thin cross section of a pant stem, mount and view it under a microscope as they have learned to do.) They should reason that they will find the familiar xylem and phloem vessels, and indeed they will. (See: stem cross sections images)

For an idealized diagram of a stem cross section see:

https://www.dreamstime.com/diagram-showing-cross-sectional-plan-dicot-stem-illustration-image163861086  (Technical names of cells beyond xylem and phloem may be overlooked for now.) 

The takeaway concept for all of this is that xylem and phloem vessels create a continuous conduit from root tips to stem tips, leaves, flowers and fruits, etc. Have kids note, however, that xylem and phloem are NOT connected into a circulatory system as are arteries and veins in vertebrate animals. Xylem and phloem function independently as described before.

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