Choosing Evidence for a Plant Needs Investigation
A second-grade class is doing a 3-week investigation: 'Do plants need light to grow?' The teacher has 24 students, 12 small identical bean seedlings, two windowsill areas (one sunny, one in a dark cabinet), and one class period per week for observations. Students will keep a simple observation log with drawings and a sentence. The teacher wants the investigation to support both a science practice (planning and carrying out investigations, analyzing data) and a literacy connection (recording observations in words). A colleague suggests adding a third condition where seedlings get light but no water, 'to make it a fair test.' The teacher is unsure and has limited seedlings.
1. Evaluate the colleague's suggestion. Should the teacher add the no-water condition? Justify with the logic of fair tests and the constraints given.
Read the worked answer
Adding a no-water condition changes the question. A fair test varies one thing (light) while holding others (water, soil, container, seedling type) constant. If you add a no-water group, you now have two variables and cannot attribute results to light alone. It also splits limited seedlings (12) across three conditions, reducing replication per condition and making the data noisier for young children to interpret.
Better: keep two conditions (light vs. dark) with the same water, soil, container, and seedling type, and use the 12 seedlings as 6 per condition (replication) so students see that a single plant's outcome isn't the whole story. If the teacher wants to address 'what else do plants need,' do it as a separate, later investigation rather than contaminating this one. The colleague's instinct about controlling variables is right; the application is not.
2. Describe how you would structure the weekly observations so students generate usable data and connect it to literacy, given only one period per week.
Read the worked answer
Use a consistent observation routine each week so data are comparable: (1) measure and record height with a non-standard or cm strip, (2) count leaves, (3) draw and color the plant, (4) write one 'I notice' sentence using a frame ('The plant in the ___ is ___ because ___'). Keep the same log page format each week so students can compare across time.
Because contact is weekly, build in a quick mid-week check (a 5-minute teacher photo or a student helper note) so students don't lose the thread, but keep formal data collection to the period. At the end, have students line up the logs and answer the investigation question using their own records, then write a short claim-evidence sentence. This links the science practice (analyzing data) to literacy (recording and explaining) without adding time.
3. At the end, the dark-cabinet plants are tall and pale, and the windowsill plants are shorter and green. A student says, 'So plants grow better in the dark.' How do you respond instructionally?
Read the worked answer
Don't correct by assertion; use the data and the concept of plant needs. Ask students to compare not just height but color and sturdiness, and to recall what the pale, spindly growth might mean (the plant stretching for light; it is not healthy green growth). Introduce the idea that 'tall' is not the same as 'healthy' and that plants make food using light.
Guide them to a claim supported by multiple observations: the dark plants grew tall and pale but were weak/not green, while light plants were green and sturdy, so light is needed for normal healthy growth. Have them revise their claim-evidence sentence to include more than one measure. This treats the surprising result as a productive data-interpretation moment rather than a wrong answer, which is exactly the practice the standard targets.