This chapter, is a pretty comprehensive look at movement of water. The first page 489 is very carefully worded. Try to see how this is so, and where the care in wording is important!
On page 490, the introduction mentions that transport of materials by diffusion/osmosis is very slow...too slow to explain movement of water or sugar from one end of the plant to another. This is a critical point. Mechanisms have evolved to accelerate movement of materials; one is cytoplasmic streaming. This process actively moves materials from one side of a cell to the other in seconds by turbulent flow (bulk flow). The other mechanism is bulk flow through "pipes" involving the Poiseuille equation parameters.
At the bottom of page 490, there are some remarkable notes on the role of evaporation in the transpiration process. These provide the scope and setting for the chapter and are well worth reading and remembering.
On page 491 under Structure of the Conducting Cells, I hope you are thinking critically when you read item #2 there. Do you have experimental evidence that argues the other way? What does an endodermis do? Do you think the roots being described here were intact (vs severed)?
At the top of page 492 in the boxed essay, look and read carefully. You know the whole Poiseuille equation. Do the figures here give an actual rate of water movement (ml/min) or do the numbers represent some rate "multiplier" to show relative flow rate? The text (top of second column) is accurate, but the table heading is wrong!
At the top of page 493, make note of the phrase in italics. This is critically important for you to understand.
In the middle of page 493 the formula for water potential of a cell is reasonably accurate providing it is at STP. Water potential in a tree must include a gravitational potential term (the footnote is correct!). At the bottom of the first column and the top of the second column less-than signs are missing before each 0.
On page 494 under Doing Botany Yourself, you might notice that you have already done this in essence with your osmosis project (the first graph you plotted).
I love the section at the bottom of pages 494, 495 and most of 496. This is what really makes this book stand out and explains why I chose it for you. I wanted you to see the "big ideas" in plant physiology, how we worked with those in the past, and how we know what we do. I wanted you to see science...not just the results of science. Here it is! It is a well-though sequence of ideas and how each one is evaluated and the model revised.
One small glitch...the data about the water potential of a leaf (-1.5 MPa) cell doesnt quite jibe with the paragraph just above the Concept bar. If you need -2 MPa, then the leaf cell needs to be at less than -2 MPa to pick the water off the top of the xylem, right?!
I really liked the information about variegated leaves on page 498. How might we test this in lab using modification of what we have already done? Think about Eosin and/or hot water.
The concept bar on this page is also important for the lab exercise...as well as the toy bird!
On page 499 the point about the biological clock is something I hope your generation of scientists can solve. I'm sure there is a Nobel prize in that solution for someone...it is a nagging problem for virtually ALL species. It must be something primitive, like our genetic heritage, as it is such a common phenomenon.
On page 501-2 there is some more real science for you to relish. I love this stuff! At the top of 502, notice the statement that the concentration of potassium ions might reach 0.5 M. That is a WOW statement. I wonder if it is correct. If you remember your osmometer results, the potato cells were about 0.35 M taking into account total solutes. Imagine if potassium alone is 0.5 M in these cells! The total osmolarity must range up towards 0.8 M!
The book talks about the mysterious importance of transpiration and mentions possible adaptive roles (503). I think the text overlooks one very important one...the evaporation of water concentrates the dilute solution of minerals coming from the roots. While the endodermis probably regulates the balance of minerals, I think evaporative losses concentrate the minerals. You have good experimental evidence of this from your Eosin Y project. I agree with the text's author that not much is made of this and more should be made of it!
I was amazed about hydrophytes using the epidermal cells for photosynthesis (504). I'm not sure of the examples that back this idea, but I don't think this is true of pond lilies. Surely it is true of Elodea, but then that plant has no mesophyll to speak of.
It would be interesting to know something about the rates of gas transport in hydrophytes with some knowledge of the structure of aerenchyma. Does the rate match consumption values? How is the exchange accomplished? Since one gas is moving downward (hopefully oxygen), is the balance gas (carbon dioxide) moving upward? How does such bidirectional flow occur, or is the carbon dioxide emitted into the water?
Go back to the Course Schedule.