THE LIVING CELL 33 
examination. ‘The lowest temperature 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 varia- 
tion than others because of special adaptations. ‘Thus, certain 
bacteria produce spores which resist exposure for an hour to the 
temperature of liquid hydrogen (—225°C.) or to that of a 
hot air oven at 100°C. Many higher plants can endure moder- 
ately 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. 
CuHEMoTROPISM (or chemotaxy) is the response of protoplasm to 
chemical stimulation. Any substances that possess the property 
of producing a deleterious effect upon protoplasm are termed 
poisons. Poisons may effect an immediate destructive combina- 
tion with living substance 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 or chemotaxis. 
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 chemo- 
taxic influences. Among flowering plants, it has been observed 
that pollen grains brought by various agencies from anthers to 
stigmas of certain plants of different species will not germinate, 
but when they are carried from one plant to another of the same 
species or variety, they readily send their pollen tubes through 
