500 Minnesota Plant Life. 



to obtain wider distribiilion. The movement is, no doubt, in- 

 duced by the force of gravity. Stimulated by this force the jelly 

 mass crawls upward. Such behavior is an excellent example of 

 an irritable response to an outward stimulus. Clearly such an 

 upward movement is distinctly different from the upward 

 movement of a balloon. The jelly mass is much heavier than 

 the air and it crawls away from the surface of the earth because 

 the force of gravity induces its mechanism to make this form 

 of response. There is a great difference between mechanism, 

 as such, and the material of which the mechanism is constructed. 

 If it were not alive it would respond to the force of gravity pre- 

 cisely as a stone does. 



Irritable behavior of plant organs. While, as has been 

 stated, all phenomena of irritability nuist receive their ultimate 

 explanation in the structure and nature of the li\ing substance, 

 yet it is expedient to separate, in an analysis, the irritable be- 

 havior of organs from that of living substance. As before, and 

 under the same conditions, the manifestations may be grouped 

 either as automatic or as induced. 



Among examples of automatic irritable phenomena in a grow- 

 ing organ, there might be named the movements of growth, 

 as they are termed. It may be discovered by careful observa- 

 tion that a stem in process of elongation does not increase in 

 length regularly, but in such a way that the tip describes a spiral 

 in the air. This growth-movement is termed nutation. It 

 may best l)e detected b\- taking, at intervals of a few moments, 

 a continuous series of photographs of some growing shoot. 

 By comparison of the different pictures it will l)e learned that 

 the stem has not been thrust straight up, but that it has been 

 nodding, first to one side and then to the other, in a highly 

 erratic manner. In part, the movement is determined by ex- 

 ternal conditions of nutriment, illumination, temperature, among 

 others, but there is also an automatic quality in the movement 

 of nutation wdiich permits it to be mentioned here. 



Another remarkable automatic movement — in this instance, 

 in a mature organ — is that of the leailets in an exotic member 

 of the pea family, known as the "telegraph plant." The leaf of 

 this plant is composed of a large central and two lateral and 

 smaller Icallets. The central leallet is (juict, but the two small 



