VEGETABLE CYTOLOGY 63 



Thermotropism is the response of living substance to the stimulus 

 of temperature. All living substance is influenced by variations in 

 temperature. Freezing disintegrates it while excessive heat causes 

 its coagulation. Active vital phenomena are therefore only evident 

 within these extremes, the limits differing depending upon the 

 endurance of the organism under examination. The lowest tem- 

 perature at which the activity of an organism becomes evident is 

 known as the minimum, that at which the activities are at their 

 best, the optimum, and the highest at which they can be continued, 

 the maximum. Some plants are able to endure greater extremes 

 of temperature variation than others because of special adaptations. 

 Thus, certain bacteria produce spores which resist exposure for an 

 hour to the temperature of liquid hydrogen (-225C) or to that of a 

 hot air oven at iooC. Many higher plants can endure moderately 

 low temperatures by the development of a hairy covering; others 

 which are killed by frost produce seeds which can endure rigid cold, 

 still others adapt themselves to existence through periods of cold 

 by passing through a latent stage in the form of bulbs, like the Squill 

 or the Lily, or rhizomes, as the Blood Root or the Hellebores. 



Chemotropism is the response of protoplasm to chemical stimula- 

 tion. Any substances that possess the property of producing a 

 deleterious effect upon protoplasm are termed poisons. Poisons 

 may effect an immediate destructive combination with living sub- 

 stance when they are called caustics, or they may have an exciting 

 or depressing effect which may eventually prove destructive without 

 visible structural change, when they are termed toxins. Caustics 

 may liquefy the protoplasm, as the alkalies, or coagulate it, as the 

 acids or salts of metals. When well diluted, chemicals may occasion 

 no destructive effects, but may call forth positive or negative 

 responses, known as positive or negative chemotropism. 



Thus, Pfeffer, working with the motile sperms of ferns, found that 

 if a capillary tube, containing a solution of malic acid be introduced 

 into water containing them, the sperms moved toward it and entered. 

 It is now generally believed that the motile male sexual cells of all 

 flowerless plants are attracted to the appropriate female sexual cells 

 by means of positive chemotropic influences. Among flowering 

 plants, it has been observed that pollen grains brought by various 



