Another attribute of a well-crafted justification relates to assumptions. The justification should be explicit about the assumptions that underlie it and even contain some speculation concerning the implications of making alternative assumptions.

Finally, a well-crafted justification for any prediction about the plant in the jar demonstrates reasoning characterized by a succession of statements that follow one another logically without gaps from statement to statement.

SCORING RUBRICS FOR DIFFERENT POPULATIONS OF STUDENTS. The plant-in-a-jar assessment exercise is an appropriate prompt for understanding plants at any grade level. Development of the scoring rubrics for students at different grade levels requires consideration of the science experiences and developmental level of the students. For instance, the justifications of students in elementary school could be expected to be based primarily on experiences with plants. Student justifications would contain little, if any, scientific terminology. A fourth-grade student might respond to the exercise in the following way:

The plant could live. It has water and sunlight. It could die if it got frozen or a bug eats it. We planted seeds in third grade. Some kids forgot to water them and they died. Eddie got scared that his seeds would not grow. He hid them in his desk. They did. The leaves were yellow. After Eddie put it in the sun it got green. The plants in our terrarium live all year long.

Expectations for justifications constructed by students in grades 5-8 are different. These should contain more generalized knowledge and use more sophisticated language and scientific concepts such as light, heat, oxygen, carbon dioxide, energy, and photosynthesis.

By grade 12, the level of sophistication should be much higher. Ideally, the 12th grader would see the plant in a jar as a physical model of the Earth's ecosystem, and view photosynthesis and respiration as complementary processes.

Setting a performance standard for a population of students depends on the population's developmental level and their experiences with science. Considerations to be made in using student responses for developing a rubric can be illustrated by discussing two justifications constructed by students who have just completed high-school biology. Student E has constructed an exemplary justification for her prediction about the plant in the jar. Student S has constructed a less satisfactory response but has not completely missed the point.

STUDENT E: If there are no insects in the jar or microorganisms that might cause some plant disease, the plant might grow a bit and live for quite a while. I know that when I was in elementary school we did this experiment. My plant died—it got covered with black mold. But some of the plants other kids had got bigger and lived for more than a year.

The plant can live because it gets energy from the sunlight. When light shines on the leaves, photosynthesis takes place. Carbon dioxide and water form carbohydrates and oxygen. This reaction transforms energy from the sun into chemical energy. Plants can do this because they have chlorophyll.

The plant needs carbohydrates for life processes like growing and moving. It uses the carbohydrates and oxygen to produce energy for life processes like growth and motion. Carbon dioxide is produced too.

After some time the plant probably will stop growing. I think that happens when all the

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