Tuesday

Raven turns the sunny jar a quarter turn so the shoots face away from the window. "Watch this all day," she says.

By afternoon Rocket is shouting. "They turned back! The tips are pointing at the glass again."

"What do you notice about where the bending happens?" Raven asks.

"Near the top," Rocket says. "The bottom of the stem did not move."

Nova hovers low over the jar. "A famous scientist noticed the same thing more than a hundred years ago," she says. "With his son."

Rocket raises an eyebrow. "Did they have a windowsill?" Nova hums. "They had seedlings. Read on."

Plants have many uses for light beyond photosynthesis. One is phototropism, a directional response that lets plants grow toward, or away from, light.

Growing toward a light source is called positive phototropism. The bending is a response to blue wavelengths of light.

In 1880, Charles Darwin and his son Francis described phototropism as the bending of seedlings toward light.

Darwin observed that light was sensed by the tip of the plant, but the bending happened in a different part.

He concluded that a signal had to travel from the tip down to the base. That is what Rocket saw: the tip senses, the stem bends.

Photosynthesis needs sunlight, carbon dioxide and water. It makes sugar and releases oxygen. Without light, that process cannot run.

A seed carries food reserves. A seedling in the dark can grow only on those reserves, because it cannot make new food.

Light is also absorbed by chlorophyll, the green pigment that begins photosynthesis. Your dark jar never had light for it to absorb.

So tall is not the same as healthy. The dark sprouts grew long, but they could not make new food.

Sharp observing. Tomorrow is Windowsill Lab: you measure every sprout, count leaves and record colors from both jars.