84 ELEMENTARY BOTANY. 



performance of the life-processes. Tropical plants live in a still 

 higher temperature. Even the thermal springs are inhabited 

 by some of the lower plants. Exposure to a freezing temper- 

 ature is fatal to many plants. Those containing much water 

 suffer most. When comparatively deficient in water— for ex- 

 ample dry seeds and spores and plants during their period of 

 rest from growth — they may endure a very low temperature 

 without injury. When freezing takes place no ice is formed 

 within the cells. The water exudes from the protoplasm and 

 escapes from the cells ; the ice crystals form between the cells 

 (in intercellular spaces) or on the surface. When the proto- 

 plasm is not killed by the freezing it will regain its former 

 turgidity and activity, provided the thawing is so slow that 

 the water may be reabsorbed by the protoplasm. 



19. If all other conditions are constant, the rapidity of 

 growth of most aerial stems is greater in darkness than in 

 light. This is due to the retarding influence of the rays of 

 high refrangibility (blue, etc.). When, therefore, the illumi- 

 nation is greater on one side of the stem than on the other, a 

 curvature arises in consequence of the retarding influence of 

 light. Thus, when plants are grown in windows they curve 

 strongly toward the light. This phenomenon has received 

 the name of heliotropism (Gr. helios, sun ; trepein, to turn). 

 It is due to the fact that the growth of the cells on the illu- 

 minated side is retarded, while on the opposite side the cells 

 elongate. 



20. The stems of most of the common plants grow upward 

 or from the earth, and the roots grow downward or toward 

 the earth ; the stems of most Mosses grow upward and their 

 rhizoids downward ; the spore-bearing or conidial filaments, 

 or hyphas, of some Eungi grow upward and the root-like 

 hyphse downward. To designate these phenomena of growth 

 the term geotropism (Gr. ge, earth ; irepein, to turn) has been 

 used. The organ is said to be positively geotropic if it grows 

 downward; and negatively geotropic if it grows upward. 

 That geotropism is due to the influence of gravitation may 

 be demonstrated experimentally by placing germinating seeds 



