How Plants Grow 45 



Things to Do and Observe 



1. To study the structure of a stem. Examine with the aid 

 of a hand lens a freshly cut cross-section of a tree at least 6 inches 

 in diameter. Note the pith in the center. It is composed of 

 thin-walled empty cells. In some woods like the oak, ash, or 

 hickory the openings made by cutting across the larger wood cells 

 may be seen. Between the outer edge of the wood and the 

 inner bark is a zone of living cells. These are smaller than the 

 older woody cells, their walls are thin, they are filled with living 

 material, and it is by the growth and division of these cells that 

 the tree increases in diameter. 



Garden plants have fewer woody cells than a tree or shrub, but 

 their general structure is the same. Examine the stem of a cab- 

 bage, noting the pith and the ring of wood. 



2. To examine living cells. If a low-power microscope is 

 available, the teacher or pupil may strip off the outer layer of 

 cells from one of the inner leaves of a fresh onion bulb, or make 

 thin cross-sections of fleshy leaves and stems of plants (for example, 

 cabbage) and mount them for examination. The leaves of 

 many mosses consist of a single layer of cells, and these may be 

 seen by examining the entire leaf with a microscope or even a good 

 hand lens. In this way one can observe the cell walls, see that the 

 living cells contain much transparent material, and that in the 

 green cells the coloring matter is in rounded masses or bodies. 

 Possibly the rounded but almost colorless body called the nucleus, 

 shown in Figures 29 and 30, may also be seen. A little 

 iodine run under the cover glass will stain the cell contents 

 and often make the nucleus visible. 



3. To observe the region of most active growth. With India ink, 

 mark off into half-inch spaces a foot of the end of a growing vine. 

 Then in a day or two note how much each space elongates. In 

 this way you can easily determine the region of most active growth 

 in the stem. 



