This chapter, is a pretty comprehensive look at roots. We don't know very much about roots, but this chapter covers quite well the field as far as it is known (or unknown).
I liked the statement (pg 333): "much of the plant's photosynthate supports an extensive underground mining operation." I wish I had thought of that way of saying it. It is the raison d'etre for roots.
In the introduction (pg 334) the author describes an alternate surface area/volume strategy for plants and animals. I hope you have given that some thought. Are there some notable exceptions? Are parts of plants using the animal strategy? Do parts or species of animals use the plant strategy? In the end the point is made that the root biomass can be as great or greater than the shoot mass, and certainly the spread can be greater. Most people haven't got a clue that there may be more underground than aboveground when looking at a plant.
Imagine a root 53 meters down in the soil (page 334) what on earth would a plant be doing expending the energy to go that deep? I think you can answer this question.
The book describes the three kinds of root systems: tap, fibrous, adventitious. It doesn't do much about biophysics to help you understand why plants might have evolved these different systems. What forces would be best resisted by a taproot? How about a fibrous system? If you were looking at a tall tree, what kind of root system might you expect to find if you were to excavate the huge volume beneath it? Have you ever tried to pull up a grass plant? What happens? How would that be adaptive? Compare that to pulling up a carrot or a beet. What is going on here? What are the selective pressures that caused these plants to evolve radically different root systems?
The book mentions that adventitious rooting is a way to clone plants (page 335-6). This discovery, made in this century, is the foundation for a huge plant nursery industry. Third largest in our state!
In listing the functions of roots (page 336), the author didn't mention movement. Later in the chapter (page 348) he does remark about contractile roots helping to pull bulb-producing plants to the correct depth in soil. In one remarkable case, Oxalis, it has been reported that contractile roots can also move a bulb up to 60 cm across the soil in one season. This is just about the extent of locomotion for a plant, but it is remarkable that an organ that serves as anchorage might also serve as an organ of locomotion. The parallel with the foot of bivalve molluscs comes to mind, however.
Also on page 336 the author states that "there is no structure in shoots that corresponds to a root cap." Now it is true that the apical dome is exposed to the air and that there is no tissue produced directly on its exterior. On the other hand, some of the functions of the root cap are carried out by tissues near the apical dome. What are some of these structures and which functions do they perform? To answer this question you need to read the section on the root cap and then read about the shoot tip, do some thinking, and recall some of your work in laboratory.
Imagine shedding 10,000 cells per day (pg 336). Is there any equivalent in humans? There must be some selective advantage to evolving such a "wasteful" process. What might it be? Or how about that hectare of corn investing in 1000 cubic meters of mucilage...what might be the reason for sending that much photosynthate into the soil? We spend millions of dollars each year in a vain attempt to try to stop the human equivalent...if we ever succeeded we would have some very unhealthy people. Why?
The author makes a good point about the zones of division, elongation, and maturation being pedagogical items that are artificial because they overlap considerably...yet in his diagram (Fig 15.7 page 338) he does not show the overlap. Maybe that's an artists error in a first edition. He does a much better job making his point in Figure 15.16 (on page 344)...too bad they are so far apart.
On page 340 the author mentions that the suberization of the hypodermis slows the outward movement of water and solutes. Do you have a related question to ask? You should! How does that fit in with the previous sentence about where the hypodermis completes its differentiation? If you can't get what I am leading you to, please ask! You should be able to think in this way, however.
In the next paragraph, it is pointed out that intercellular spaces may occupy as much as 30% of the root's volume. Why on earth might that be so? What is the root trying to do? If you are trying to penetrate something as hard as soil, why be spongy? Or is there some compromise that is being struck? Are we looking at a developmental process? How did the onion root tip look in terms of intercellular space?
The author mentions that the endodermis has Casparian strips and describes their position and function quite well. Why would evolution have arrived at this solution? What is the root accomplishing by forcing solutes and water to pass through cytoplasm? It must be pretty important! Duh! Like only the most important function in roots. I don't need to tell you, right? I'm surprised the book didn't spell this out for you. Maybe the author felt you could figure it out too?
After the endodermis becomes completely suberized (as far as it does), where might you expect to find passage cells? When you go to lab, look for them in the mature root slides. They are quite easy to find and they are right where you should expect them to be!
In mature roots, the parenchyma often gets lignified too. When you examine the mature root slides in lab, look for that. It is shown in figure 15.11c on page 341 in the pericycle. Why on earth would plants lignify the parenchyma?
One of you might be interested in the Cephalosporium infection idea for an independent study.
On page 342 the author writes "the final unique feature of roots..." In reading the chapter did you make note of these? How many did you find? Is your list complete? Maybe you should put them together into a list!
The text alludes to an important development of secondary growth (cambial meristem derivatives maturing to add to the plant body). This is important for most, if not all, woody plants (vines, shrubs, trees).
On page 343 there is a small error. The label "Metaxylem" should be "Protoxylem." They'll probably fix that in the next edition.
On page 344 did you catch the figure of 1010 organisms per milliliter of soil? Yes, I translated cm3 to volume to drive the point home for you. Think about that number...10,000,000,000 per milliliter. How big can each individual be? What are these organisms? Find out on page 345.
Also on page 345 the author has a paragraph on anchorage. His concluding sentence talks about branch roots. Would this add to the anchorage function? How?
On page 346 there is a paragraph illustrating two different strategies for growing roots in dry soil. How would these plants be different in other ways? How would their root adaptations support/cause different shoot structures and functions. Which one is most likely a CAM plant, for example?
After such a nice section on the rhizosphere being such a nice "soup" for growing microorganisms, there is this sentence about "phytoalexins and other obnoxious chemicals." What do you make of this? Is the author talking out of both sides of his mouth? Hmm, maybe this is an area needing some more information. Maybe your research career will shed some light on this root function?
Do differential responses to the gravity vector surprise you? Why, why not? How might this be accomplished? What might be the mechanism for differential gravity responses? Hey, you too can win the Nobel prize if you are clever enough!
On page 348, in the paragraph on storage you see a long list of plants that store starch in their roots. How about the inclusion of Beta vulgaris in this list? Is starch the only storage molecule found in beet roots? A white variety of Beta is really important for inland temperate zone inhabitants because is does not store starch primarily. Which one is that? Why would that beet not be as important for people along the coast, such as here in Connecticut?
By the way, the more refined name for "suckers" is root sprouts. And "disrespectful" fits nicely here since this is the foundation for a plant propagation industry.
Figure 15.20a (top) shows a very colorful view of root nodules and the caption attributes the color to leghemoglobin. Why would a root nodule have this compound in it? It sounds very "expensive" for a root to make (iron bound in protein), so it must have a very important function. It likely relates to nitrogen fixation by the bacteria in the nodules. Maybe you can think about the function of hemoglobin in humans and relate it to use by plants housing bacteria to perform nitrogen fixation.
The discussion on the velamen (page 350) shows how our knowledge has advanced. I got my PhD in 1981 and back then we taught the first part of the paragraph as "gospel." Obviously we have learned something about roots since then...of course notice the words "appears to be" in the last sentence. If you get the signal, your mind should be saying "factoid."
Notice photosynthesis as a root function on page 350 at least for some plants...which ones? How could a cell without chloroplasts do photosynthesis? I mean, all the roots you have seen in lab are pure white! Would the roots of, say, a corn plant do photosynthesis if given the opportunity? How might you design an experiment for an independent study to answer this very question? You already know the techniques needed (believe it or not!).
Here is another project someone might like to try. On page 334 the text indicates that roots have an apical dominance function that is similar to that in shoots. Gosh, I haven't seen much in the literature about that. Did I miss something? How would you design an experiment to test that? Remember observing and marking a corn primary root at 1 mm intervals? Here is a very nice project for someone to do...and you don't even have to get your hands dirty to do it! Remember the paper towel? Figure 15.19 shows you another interesting project.
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