Rocket holds a leaf from the course seedlings up to the window. Light glows through it.
"This is just one green thing," he says. "Where is the ladder in a leaf?"
Raven hands him the hand lens. "What do you notice up close?"
Rocket squints. "Tiny shapes. Rows and rows of them. Cells, I guess, in layers."
"Layers of cells make the leaf," Raven says. "Plants have organs too. It said so on Monday."
Nova brightens. "And the stomach? And the whole forest outside?" she asks. "Where do they sit?"
Rocket grins. "Organ. Organism times a thousand." Raven writes "levels everywhere" across the top of the page.
Organs are present not only in animals but also in plants. A leaf is built from many layers of cells.
The leaf has a top side and a bottom side that are different. A middle layer sits between them.
So a leaf fits the ladder: cells form layers, layers build the leaf, and the leaf belongs to the whole plant.
Each tree in a forest is an organism. A forest holds thousands of organisms side by side.
The digestive system consists of several organs, including the stomach, intestines, liver and pancreas.
The walls of the stomach and intestines contain smooth muscle tissue. It squeezes without you deciding to.
Cells make smooth muscle tissue. The tissue helps build the stomach, an organ. The organs form the digestive system.
Complex animals have four primary tissue types: epithelial, connective, muscle and nervous. Every organ is a mix of them.
| Body part | Level | Example tissue inside |
|---|---|---|
| Cardiac muscle cell | Cell | |
| Smooth muscle | Tissue | |
| Stomach | Organ | Smooth muscle |
| Heart | Organ | Cardiac muscle |
| Digestive system | Organ system | Several organs |
| One tree, one person | Organism | All of them |
| Week review | True or false? |
|---|---|
| All living things are made of cells. | ? |
| A tissue is a group of similar cells with the same function. | ? |
| The heart is a tissue. | ? |
| Cardiac muscle is found only in the heart. | ? |
| A resting pulse is counted after at least ten minutes of rest. | ? |
Your pulse rose after twenty jumps. Your breathing probably sped up too.
Next week you will find out why those two numbers rise together, and how the lungs, heart and muscles share one job.
Strong week, biologist. Tomorrow is Nature Day: a family walk to find organisms and count a walking pulse.