How Plants Grow 37 
are the new parts formed? Where in a plant is 
growth most actively going on? ‘To answer these ques- 
Fic. 29. Diagrammatic representation of a group of plant cells. Above is 
a cell with the wall cut and a part of it lifted to show the contents. Below are 
cells cut crosswise and lengthwise. All plants and animals are built up of these 
cell units; but, of course, many kinds of cells are very different from the box- 
like structures here shown. 
tions it is necessary to explain the structure of a plant. 
The cells of a plant. If a thin section or piece of a 
plant is examined under a microscope, it is seen to be 
made of tiny units that are called cells. Each cell, like 
a loaf of bread, has length, breadth, and thickness. The 
outer part of the cell is a wall, and if the cell is living, 
within the wall is a soft, living material that appears 
somewhat like white of egg. In a full-grown plant cell 
the most active part of its substance is found chiefly 
in a layer just within and completely lining the wall, 
in strands of this material that extend from the lining 
layer to a rounded body called the nucleus, and in the 
