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 (-2 2 5°C) or to that of a 
hot air oven at ioo°C. 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 
