22 INTRODUCTION TO BOTANY 
plant consists of hundreds of thousands or millions of cooper- 
ating cells, which together carry on the work of the plant. 
The root-hair cell shows its life most clearly by growing and 
making its way around obstacles (fig. 6). Other plant cells 
often give much more striking evidence of life in the move- 
ments which are executed by single cells or by organs built up 
of great numbers of cells. Many such movements will be 
described in subsequent chapters. In this place it is sufficient 
to call the student’s attention to the fact that the protoplast 
is alive in just the same sense that any minute animal is alive. 
Whatever any living organism can do it does by virtue of the 
energy of its protoplasts. 
The most remarkable and peculiar of the characteristics of 
protoplasm are due to its possessing érritability. By this is 
meant the power to respond in some definite way to any suit- 
able stimulus, or exciting cause, acting from within or without 
the plant body. Some of the principal stimuli are gravity, 
heat, light, chemical substances, and contact with solid objects. 
When protoplasts, either singly or combined into some organ 
of the plant, are acted upon by any stimulus to which they 
are sensitive, there is usually no immediate visible response. 
If, for example, a young seedling with a stout taproot is pinned 
horizontally to a piece of cork which lines the vertical side 
of a glass jar containing moist air, no change is at first noticed. 
In a few hours, however, the part of the root for a short 
distance back of the tip will be found bending vertically 
downward. This movement is a response to the stimulus of 
gravity, acting upon the very sensitive young root and caus- 
ing unequal growth in the upper and lower sides. It is not a 
mere bending, like that of an unsupported piece of wet string, 
for the moving end-portion of the root will be found to push 
downward with a force of more than ten times its own weight. 
19. Turgidity. Root hairs and other cells of plants usually 
take up water until the cell walls are distended with water 
anil protoplasm. The outward pressure which distends and 
stretches the walls is called turyor, and the resulting condition 
