252 ❯ STEP 4. Review the Knowledge You Need to Score High
will be assigned a single variable. Take the weights of your little guys as soon as they’re all
ready to go; this will be your time zeroweights. Then measure the weights again 24 hours
later, for as many days as your teacher indicates. Remember that if a leaf falls off during this
experiment, it has to stay with the plant for subsequent weighings. When your data collec-
tion is over, you need to determine the best way to compare results between treatments.
Just looking at the total change in weight doesn’t really get you anywhere; it’s hard to com-
pare if the initial plant weights were different to begin with. Calculating percent change
would help. But what if you need to determine the total surface area (cm^2 ) of your leaves?
The idea of outlining each and every leaf on a piece of grid paper makes even the most ded-
icated AP Biology student weep, so here’s a suggestion: Calculate the surface area for only
one lucky little leaf, and then weigh it. Now you have a conversion ratio (SA/g) you can
use to determine the (approximate) surface area for all the leaves. All you need to do is
determine how much all those leaves weigh. Easy!
Another method of measuring water loss is by using a device called a potometer. This
tracks transpiration from only part of a plant that has been inserted into a water-filled tube
with a pipette stuck on the other end. As water evaporates from the leaves, the water is
pulled down the pipette, allowing you to track changes in volume.Results
Ideally, any treatment that increased water loss through either more photosynthesis (e.g.,
light) or more evaporation from the leaf surfaces (e.g., wind) would decrease your plant’s
weight. On the flip side, if you slow down transpiration by creating a humid environment
(e.g., misting your plant) or slow down photosynthesis (e.g., stick the poor plant in the
dark), you would expect water loss to be slowed.Key Concepts- The higher the leaf surface area, the greater the rate of transpiration.
- The more stomata, the more water loss.
- An increase in water potential of the environment would slow evaporation; a decrease in
water potential would increase evaporation.
Investigation 12: Fruit Fly Behavior
This experiment draws on information found in Chapter 17, Behavior Ecology and
Ethology. This experiment is basically an exercise in messing with fruit flies’ heads. You get
to stick them in a choice chamber and explore environmental factors that either attract or
repel them.Basic Setup
First, get to know your flies. For example, males have a darker abdomen whereas females’
abdomens are pale and rounder. Next, you get to create something called a choice chamber.
In this experiment the chamber consists of two plastic bottles with their bottoms cut off
and joined together with tape. Each end of the chamber will have one of the substances you
decide to test. You can also use the chamber to investigate the flies’ response to other vari-
ables, such as light or gravity. No matter what you’re interested in, wrangle about 25 of
those little creatures in there and let them get used to their new digs. Place a cotton ball
with a few drops of water in the cap at one end (control) and a few drops of the substance
you’re testing at the other end. The flies will exhibit positive chemotaxis if they move
toward the substance you’re testing, negative chemotaxis if they move away from it.KEY IDEABIG IDEA 4
Interactions