322 TEXTBOOK OF PLANT PHYSIOLOGY 



The data indicate that the amount of light required to produce 

 reaction is a constant quantity, while the limits of fluctuation 

 shown by light intensity are very great indeed. This important 

 deduction is named the "law of the quantity of stimulus." Fur- 

 ther research has shown that this law can be applied not only to 

 phototropism but also to many other phenomena of stimulation. 

 Moreover, in human physiology the same law is known to hold true 

 for the reaction of the human eye to light. 



95. Geotropism. — One may observe in higher plants a definite 

 orientation of their organs in respect to the vertical line. The chief 

 axis of a plant, its stem and main root, acquires a vertical position, 

 the stem growing upward and the root downward. The lateral 

 axes of the first order are situated at a certain acute angle to the 

 stem or root, the axes of the second order are again situated at a 

 certain angle, and so on. Leaves, on the other hand, providing 

 they are equally illuminated on all sides, are usually adjusted in a 

 horizontal plane. This distribution is usually characterized by 

 great regularity, particularly in plants of the more primitive types, 

 such as, for instance, the conifers. The regular colonnade formed 

 by a forest of pines and the strictly parallel rows of sunflower and 

 cereal stems are pictures familiar to all, as are also the strictly 

 vertical position of the trunks of gigantic cacti (Fig. 76). 



This regularity in the distribution of the axial organs of plants 

 is naturally attributed to the force which always acts in a 

 strictly vertical direction, namely to gravity. The capacity of 

 the plant to acquire a definite position with regard to the vertical 

 line, therefore, has been called "geotropism." As in the phenome- 

 non of phototropism, three things are distinguished — positive 

 geotropism of the roots, which grow downward; negative geo- 

 tropism of stems, which grow upward; and transverse or dia- 

 geotropism of leaves, which remain in a horizontal position. A 

 definite proof that gravity is the determining factor in geotropism 

 was supplied by Knight in the beginning of the nineteenth century. 

 He succeeded in solving this veiy difficult problem by means of 

 an ingenious procedure. The difficulty of finding sufficient proof 

 in this instance was due to the fact that, because of our existence 

 on the earth's surface, we are unable to escape the effect of the force 

 of gravitation, and therefore cannot apply so simple a method as, 

 for instance, the protection of a plant from light, which is used 

 in studying phototropism. Not being able entirely to remove the 



