REPRODUCTION AND DEVELOPMENT OF PLANTS 285 



Experiments on bean seedlings placed in a horizontal posi- 

 tion in the dark show that the stems turn and grow upward, 

 while the primary root turns and grows downward. This 

 shows that it is gravity and not light which gives direction 

 to the growth of stems. 



Some parts of plants may respond to gravity by growing 

 in a horizontal direction or at' any intermediate angle, as is 

 shown by the branches of the main stem and by secondary 

 roots. Thus gravity seems to act as a pointer or directive 

 force, so to speak. More rapid growth on one side of the 

 stem or branch is the power that sends that part of the 

 plant into the required position. 



327. The Influence of Water. Whenever any external 

 force or substance is important to the life of a plant organ, 

 the organ develops a corresponding sensibility to the influ- 

 ence of that force or substance. Thus roots respond to 

 water. If water is evenly distributed, roots develop equally 

 on all sides; otherwise roots grow only in the direction of 

 greatest moisture. An example is found in the fact that 

 roots of trees often enter and block up tile drains laid in wet 

 ground, long distances from the tree itself. 



In watering house plants, water should be placed in a 

 saucer under the pot rather than on the surface. The roots 

 turn toward the water, and if that brings them near 

 the surface, they are more likely to become dry between 

 waterings. 



328. The Influence of Contact. Irritability of proto- 

 plasm to contact with objects is almost universal. 

 Tendril climbers, such as the pea, a near relative of the 

 bean, make use of this irritability with the result that 

 they attach themselves to supports. In this way they are 

 enabled to elevate their leaves and flowers into more favor- 

 able situations. When the tendril touches the solid body, 

 growth on that side is stopped, while growth on the opposite 

 side is increased and the tendril curves. 



