The first part of this chapter, Pages 305-311 is a look at the structure of stems. The remainder of the chapter applies to leaves, those appendages on stems. Those are studied in a few days from now.
The presence of leaves divides the shoot apical meristem into multiple and disjunct regions of cell division, elongation, and maturation. The concept of node being a segment of the stem's length where a leaf (leaves) are attached is not particularly well explained. Why do we call this part a node? A node is an area where things are not happening.
For example, in our lymph systems a node is a place where a cell being transported through the lymph ducts can stop and rest in its journey. Transport is not occurring there.
For a second example, a rope is being swung (as in jump-rope) in such a way that there are a series of loops being swung along the length of the rope. It looks like there is a standing wave in the rope. Of course the peaks and valleys are symmetrical and the rope is being twisted from peak to valley and swinging a wide arc. Between the waves the rope is not swinging. It is merely turning on its axis. This location where swinging does not occur is called...you guessed it...a node.
In the case of a stem, the place where leaves are attached is also called a node. What is not occurring significantly there is elongation growth. Since the leaves attach here, elongation of the nodal tissue would pull apart the connection for the leaf stalk (petiole). Evolution has therefore arrived at a nodal (no growth here) solution.
The internode of the stem is, of course, the length of stem between nodes. This part of the stem is capable of considerable growth. Your books lists some amazing examples including bamboo. Growth to lift leaves into the light and out-compete neighboring plants is a critical function provided by stems.
Evolution has also provided intercalary meristems in certain plants that have been subjected to the selection pressure of grazing by animals. Your book rightly ascribes much of animal livelihood to these meristems. Without them the food available for grazers (and grazers available as prey for carnivores) would be severely reduced. Our food web relies upon these meristems for a great deal of its productivity.
One question that would immediately come to mind is whether roots also have nodes and internodes. Clearly roots have lateral roots at intervals along their length. Would it make sense to call the location of lateral attachment a node; should the length of the root between laterals be called an internode? Taking into account the concept of a node being a place where something does not occur (but that does occur in the other areas), think about this and try to come up with a suitable defense for your answer.
The diagram (Fig 14.1) has the line for "node" ending on the base of the petiole of a leaf. Would this be a correct placement? Perhaps you can mark your book with a better line. The bud on the leaf above this could have a more specific name....what would that be? Is there a synonym? There is also a structure labeled "shoot tip." What are a few synonyms for this structure?
On page 308 the vascular tissue is described. The xylem and phloem are said to occur side-by-side...in other words sharing one side...we could call this a collateral arrangement of xylem and phloem. I am likely to use that term in lecture.
` Later on that page the author correctly warns you that "these generalities are not absolute..." Here he is being candid with you about fact and factoid. If you learn nothing else from me, I hope you will have learned that science has no proofs because of chance and exceptions. For every "fact" there is an exception or chance deviation that makes it a "factoid."
By the way, the author, like many other authors, leads you into a lousy thought with the phrase "vascular bundles scattered throughout the stem." So many people look at a complex pattern and say "scattered" out of frustration. The pattern is usually exceedingly regular, but complex, so that one might think of it as random because of our inability to resolve the complexity by simple observation. It is doubtful that evolution ever comes up with such capricious outcomes as bundles "scattered." They would be like Tevya described in his song in Fiddler on the Roof: "One long staircase just going up, and one even longer going down, and one more leading nowhere just for show." The whole thing makes no logical sense except that Tevya, a poor man, was describing his house in his fantasy of being a very rich man. It was something he did not know much about and so his concept was flawed and his description with it. If we study carefully the patterns of vascular bundles in monocots and Bougainvillea I'm sure we won't find them randomly scattered "leading nowhere, just for show!" I'd rather that you think of these bundles a having a definite, but complex, pattern. Those who study such things (especially Jim French) know how long the stairways are and where they lead. The bundles are all connected as shown in videotapes of countless serial sections through whole plants!
The pattern in many monocots is a simple ring of stem vascular bundles (just like dicots). Bundles that extend into the bases of the leaves as parallel veins depart from the main ring of bundles perhaps several nodes lower on the stem. Because any plane of section, then, has several concentric rings of leaf bundles in addition to the main central ring of stem bundles, it looks pretty complicated. But if you think of concentric rings of bundles headed into progressively higher leaves, the complexity becomes understandable. If you take that thought into account as you look at Figure 14.5B, you will understand what is going on.
In fact, if you look at Figure 14.5A with the concept in mind, you will begin to wonder which of the bundles in the "one" ring are really stem vascular bundles, and which are branching bundles that eventually go out into the next pair of leaves at the node just above! If you look, you can probably figure it out! Can you tell that the plant sectioned here has opposite leaves? It does!
In figure 14.6, the "young leaves" label will likely be called "leaf primordia" by me. Similarly "shell zone" sounds more like a beach than an axillary bud primordium.
Plants with strong apical dominance have canopies that are usually cone-shaped or columnar (I don't know what "tiered, Christmas-tree shape" means). Plants with weak apical dominance branch profusely and their canopy ends up being very bushy...spherical or perhaps even like an inverted cone (I guess that is "shrub shape").
Some of the modified stems will be seen in the greenhouse. Be sure to ask me to show you some of them in lab. I've never heard of "searcher shoots" as a term (pg 310), but certainly many twining plants show circumnutation (a growth movement in which the terminal part of the stem rotates at a slow speed about the stem axis until it touches an object in the environment. Time-lapse movies of this are wild! It looks like the plant is "whipping" its stem around and around until it hits an object to climb. Then it wraps its way up the object with further circumnutation.
I don't like the word choice for potato tubers at all. The potato tuber is produced at the end of an underground stem. The name for an underground stem is a rhizome (see two paragraphs above in text) not a stolon. Furthermore "burrow" calls up completely wrong imagery in my mind. This underground stem does not behave like a badger or prairie dog. It is at best a naked mole rat (living always below ground) but its passage through the soil is really much more like root growth than appendage-driven digging. The author described root growth quite well, so I hope that is the image you will bring to the front of your mind here. The burrowing image works better for me in describing the gynophores in peanut, even though imperfect there also.
This part of the chapter closes on page 311 with some economic ideas. Indeed there are wonderful examples described here. For Willimantic, CT, the one about stem fibers of the flax plant were once our main economic commodity! Our town was the world leader in the production of linen thread and particularly linen fabric!
There is one example of a stem that you eat and, while potato is a specialized example, this one is much more ordinary. It is Asparagus. The plant is a monocot and next time you have a spear on your plate, look at the cut end, or make a fresh cut and see if you can tell how its bundles are arranged. Looking at the leaves (tiny scale-like objects), can you see evidence of the monocot leaf form (more on that later)? If you are "picking" at your Asparagus at dinner, maybe your parents will think you actually are going to eat it. [I like it very much, but it wasn't always that way! Fortunately my parents couldn't afford it more than about once a decade.] Another exotic stem vegetable is kohlrabi (Brassica oleracea caulorapa).
Finally, given what you know about plants, can you tell that the potato in Figure 14.8B was posed? Not everything shown could have formed naturally in an undisturbed soil setting. What are your clues? Here are some hints: think about osmosis, spring soil conditions, cleanliness, gravitropism, etiolated growth, and stem greening in light.
As long as we are criticizing the book so much, we might as well turn to Figure 12.1 on page 263. Can you see several flaws here? Based on your knowledge of plant morphology you are looking at a dicot with alternate leaves. But check out the leaf primordia...what's wrong here? Then with your knowledge of plant anatomy look at the three cross-sections just to the right. A is plausible (as you will soon learn). B has one flaw that tells you it could not come from the plant diagrammed...think three-dimensions to understand why. Finally cross-section C is just from the wrong class of plants, right?! How would you describe the plant from which the root cross section was taken?
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