PROHYDROTROPISM OF ROOTS 67 



from it in certain instances in which dryer atmosphere or soil is 

 desirable. Roots, pollen tubes and rhizoids are found to be pro- 

 hydrotropic, while the sporangia of some of the fungi and myxo- 

 mycetes are apohydrotropic. The latter reaction, which is shown 

 by Mucor stolonifer, Phycomyces nitens and Coprinus velaris, is 

 evidently an adaptation for carrying spores as far as possible from 

 the moist surroundings of the plant, and permitting their dissem- 

 ination by the wind. Roots placed in currents of water gener- 

 ally react to the force of the flowing water by bending toward or 

 away from the source of the current. Similar reactions are shown 

 by plasmodial masses of the myxomycetes. The curvature in 

 response to a current of water is probably due to the mechanical 

 force received, rather than to any quality of the water and is 

 termed rheotropism. 



104. Prohydrotropism of Boots. Cover the outside of a small 

 glass funnel, or one of porous earthenware with wet filter paper. 

 Stop the opening in the funnel with a plug of cotton wool and 

 fill with moist sand, rounding it up on top. Imbed kernels of 

 corn around the edge of the funnel in the sand, directing the 

 apices of the seeds outward and over the edge of the funnel. 

 Cover the kernels with a circular piece of filter paper. Place the 

 funnel in the mouth of a bottle filled with water. Set the prep- 

 aration under a bell-jar. As the corn grows the roots will escape 

 over the edge of the funnel into the air. If the air in the bell-jar 

 is saturated with moisture the roots will grow directly downward. 

 If the air has been kept only moist enough to prevent desic- 

 cation however, the roots will bend toward the filter paper, the 

 lower edge of which is immersed in the water in the bottle. 

 The proper degree of ventilation may be secured by raising one 

 edge of the bell-jar. This reaction is a delicate one and a second 

 or third trial may be necessary to adjust all the conditions prop- 

 erly (Arthur, Exp. in Veg. Physiol.). The sand may be re- 

 placed by a shallow germinating dish in which the seeds are 

 covered with filter paper, and openings are provided for the exit 

 of the roots (Fig. 28). 



