Biology of Higher Plants — Anatomy and Physiology 243 



vironmental conditions which cause a plant to react are known as 

 external stimuli. A reaction to a stimulus which possesses direction is 

 known as a tropism. The following tropisms are common in plants: 



Phototropism (Reaction to Light). — The stems usually grow toward 

 light (positive phototropism), while roots usually grow away from light 

 (negative phototropism), 



Geotropism (Reaction to Gravity). — Stems are generally negatively 

 geotropic, while most roots are positively geotropic. 



Chemotropism (Reaction to Chemicals). — This reaction is exhibited 

 by plants in various ways, depending upon the quality and quantity of 

 the chemical and the species of plant. 



Thermotropism (Reaction to Heat). — Certain plant structures grow 

 toward heat, while others grow away from it, depending on the quan- 

 tity and quality of the heat and the species of plant. The reaction of 

 cold (the absence of heat) is also important and characteristic. 



Hydrotropism (Reaction to Moisture). — Roots tend to be positively 

 hydrotropic, or grow toward a supply of moisture, because one of their 

 functions is to supply water to the plant. 



Thigmotropism (Reaction to Contact With Solid Objects). — The 

 small tendrils of certain plants are stimulated by contact with solid 

 objects so that the tendrils grow around that object. This contact 

 stimulates the cells of that particular region so as to produce an un- 

 equal rate of mitosis in the two sides of the tendril. This unequal rate 

 of growth results in the curving of the tendril around the solid object. 



A young stem always bends toward the light because of a greater 

 concentration of growth hormone on the darkened side of the stem. 

 One possible explanation for this is that it is at least partly due to a 

 light-induced change in the electric potential across that stem. In a 

 similar manner the various tropisms are thought to be determined and 

 influenced by the actions of the various hormones present in the plant. 

 This is known as the hormone explanation of tropisms. 



Xni. PLANT PIGMENTS 



The structures and functions of pigments in the plant kingdom are 

 not well understood at the present time. There is no doubt that pig- 

 ments play important roles, but only future experiments in this field will 

 reveal their true significance. It is commonly known that a variety of 

 pigments exist in leaves, flowers, seeds, stems, and fruits. Certain uni- 

 cellular and simple multicellular plants have pigments whose functions 



