Minnesota Plant Life. 503 



the air in which it may be suspended. Some organs place 

 themselves against a stream of water, while others grow with 

 the current. Some organs bend towards the positive electric 

 pole, others towards the negative. Growth in a magnetic field 

 shows that magnetic force has its directive influence. Contact 

 induces curvatures. Root tips upon touching a hard body be- 

 come convex toward the body that touches their sensitive 

 area. Tendrils, on the other hand, and twining stems be- 

 come concave toward the body that touches their sensitive 

 surfaces, thus enabling themselves to seize hold of supports. 

 A wound often induces curvature. Thus, branding a root tip 

 with a hot wire induces it to curve to one side. In general, the 

 plant is a sensitive creature feeling easily slight changes in the 

 surroundings and accommodating itself to them, so far as pos- 

 sible, by irritable curvatures. 



Special organs of irritability are present in many plants. Of 

 these the pulvinus of the pea family and the tendril of gourds 

 or smilax, might be cited as illustrations. At the base of each 

 leaflet of a bean plant, for example, may be noticed a short sec- 

 tion of the leaf stem in which the color is deeper green than 

 elsewhere. A similar section exists at the base of the main 

 leaf-stem. Such an area is called a pulvinus and is the motile 

 organ of the leaf. When provided with pulvini, as are beans, 

 clovers, wood-sorrels and several other plants, the leaves easily 

 change their positions under slight stimuli. Clover leaves ex- 

 amined at night will be found to be in sleep-positions different 

 from those of the day, and doubtless induced by falling tem- 

 perature and less illumination. Leaves with pulvini commonly 

 make automatic movements, most noticeable in the telegraph 

 plant mentioned above. Some leaves with pulvini are highly 

 sensitive, and of this condition the well-known Mimosa is an 

 example. In this plant a slight shock or burn causes the 

 numerous leaflets to fold together and the whole leaf to drop 

 downward on its stem. The rapidity with which the action 

 takes place makes it very striking, but it is not particularly 

 different, except in amplitude and celerity, from many other 

 sensitive reactions. All plants are sensitive plants, if one 

 watches them closely enough. The curious Venus's fly-trap of 

 Carolina is another quickly-moving variety. Its leaf-blade may 



