404 READINGS IN BIOLOGICAL SCIENCE 



pole. When starved, Amoeba becomes more active than usual, while after 

 a heavy meal it becomes sluggish. 



One must not however be too sure of the simplicity of an Amoeba. While 

 to the eye of the microscopist it appears as an "almost structureless mass of 

 jelly," nevertheless the complexity of the molecules composing this jelly 

 is such as to defy analysis by the most skillful chemist. And even were it 

 possible to obtain an exact analysis of the Amoeba molecules, the number 

 of atoms composing the latter is so great as to render possible several million 

 combinations of these atoms, each in a different way and each possibly re- 

 sponsible for every new response which it makes to its surroundings. 



The ability of higher plants to respond to stimuli is a matter of common 

 knowledge. We place a plant in our window and soon leaves and stem are 

 bending toward the light. The compass plant is a devoted 'worshiper' of 

 the sun. In the dawn it turns its opening flowers eastward to greet the rising 

 sun, while at eventide they face the west attendant upon its setting. The 

 mold Filohohis grows upon horse manure. When its spores ripen they are 

 thrown by the plant with considerable force, surrounded by the spore 

 cases, in the direction of the light. If a little fresh horse manure be placed 

 in a box with a small window, the filaments of the mold turn toward the 

 window, and as the spores ripen they are thrown in their cases against the 

 window to which they adhere. A tree is felled by a land-slide or a tornado 

 and some of its roots are left embedded in the ground. Soon the young 

 flexible branches turn and grow upward opposite to the direction of grav- 

 ity. Roots, on the contrary, when placed in a horizontal position, or in- 

 verted so as to point upward, will soon respond to the pull of gravity and 

 grow downward. A seedling is suspended with its rootlets immersed in a 

 stream of water, and soon they bend and grow against the current of the 

 stream. Touch the leaves of the Mimosa or sensitive plant and almost im- 

 mediately the paired lobes of the leaflets fold together and the leaf it- 

 self droops slightly, soon however resuming its original position if undis- 

 turbed. 



Can these responses of the unicellular animals and plants be explained 

 on a physico-chemical basis? This the leader of the mechanist school in 

 America, Jacques Loeb, endeavors to do with his "forced movement" or 

 "tropism theory." According to this theory every organism is in a state 

 of physiological equilibrium or balance with respect to a median plane 

 of symmetry, until it is subjected on one side or the other to a stimulus, 

 such as heat, light, electricity, etc.; which stimulus induces certain physico- 

 chemical changes, differing in degree on either side of the body, this dif- 

 ference forcing the organism to respond unequally on the two sides, and 

 then perform a "forced movement" or a "tropism" (turning). The stem of 

 a plant turns toward the light, or bends upward, because of a difference 

 in the amount of chemical substances * on the two sides, and "this causes 



• These are now known as plant auxins, hormone-like substances. — Ed. 



