Chapter 1. The Development of Trees

Introduction

Topics in Botany

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You must read each slide, and complete any questions on the slide, in sequence.

The Development of Trees

Author: Tucker P. Hamilton

Stanford University

Synopsis

In this activity, you will learn about secondary growth.

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1.1 How is Secondary Growth Important?

As a plant grows, its size requires more support and vascular pathways. The plant is developing a large amount of leaves that need a large amount of water and minerals from the roots. So, in order to do this, the plant creates more xylem to conduct more water. It will also create more phloem to move the carbohydrates from photosynthesis from the leaves to the roots.

Each year, the vascular cambium produces a layer of secondary xylem (towards the inside) and a layer of secondary phloem (towards the outside), resulting in an expansion in girth (diameter). The secondary xylem that is produced differs throughout the year (i.e.: spring and summer wood). Spring wood cells are generally thin-walled with a large diameter, compared to summer wood, where the cell walls are thicker and the diameter is smaller. The rings you see when looking at a cross section of a tree is actually secondary xylem. The “line” in-between summer and spring wood is caused by the almost complete lack of growth during the winter. Since a “ring” seems to occur once a year, they can be used to provide an age when counted accurately. Secondary phloem, on the other hand, cannot be used to determine the age of the tree. Secondary phloem, along with the periderm, is what we normally call “bark” on the tree. This “bark” is sloughed off regularly as the tree grows and expands in diameter, or when a mean person rips it off for fun. So, again, you cannot count phloem to determine the age, but you can count xylem to do so.

Figure 1.1: This shows the key terms for secondary growth.

Question 1.1

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Correct. Great job!

Incorrect.

1.2 Transpiration

Transpiration is the evaporation of water from plants. It occurs mainly at the leaves while their stomata are open for the passage of carbon dioxide and oxygen during photosynthesis.

This process has a good side and a bad side. The good side is that it brings water and minerals up from the roots and into the leaves. It acts as an “engine” to pull these substances up against the force of gravity. However, the bad side is that when the stoma are open, water loss can occur and can dry out the plant.

1.3 Beyond Hunger: Humans in a Land of Plenty

Source: The Big Hurt by CWK Network/Connect with Kids (www.connectwithkids.com)]

It would be great to have a signal system that changed our taste preferences. A possible interior monologue would go something like this:

“Looks like there is plenty of food and there is going to be abundant food in the foreseeable future. The layer of fat in my body is at an optimal level. The regulatory system and the sensation/perception system regarding taste and smell will now change so that foods high in sweets and fats will be rejected as not very appealing foods.”

Unfortunately, it doesn’t work that way. Foods that are no longer beneficial are still appealing.

Play the video to listen to some older children and their parents discuss food choices and some problems that overweight children face.

Question

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Children and adults sometimes eat unhealthy foods as a reward after a stressful day or a special event. Foods may be associated with certain activities, such as popcorn or candy with watching a movie. Adults often feel time pressures and may settle for unhealthy food choices because these foods can be purchased and eaten quickly. Because adults have more control over food choices, they can decide to make a commitment to increase healthy foods and decrease fast foods. Unfortunately, a 10-year-old cannot do this without parental support!

1.4 Check Your Understanding

Question

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Question

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Question

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Question

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Many poisonous plants have bitter tastes. Those who ate them often died before reproducing. Babies scrunch up their faces in disgust when bitter tastes are placed on their tongues. Babies like sweets. All humans are born with a preference for sweet and salty tastes. Those who built up stores of fat survived during famines.

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REFERENCES:

[Center for Disease Control] CDC Grand Rounds: Childhood Obesity in the United States. URL: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6002a2.htm?s_cid=mm6002a2_w Retrieved 3/31/11

Collins, A. & Peebles, R. (2011). Pediatric obesity: A pediatrician’s viewpoint. In Debasis Bagchi (Editor) Global Perspectives on Childhood Obesity: Current Status, Consequences and Prevention. Pages 257-264.