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 irritability. 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 

 and protoplasm. The outward pressure which distends and 

 stretches the walls is called turgor, and the resulting condition 



