160 THE PLANT WORLD 



investigations on the toxic action of ' ' ions ' ' on protoplasm were to lead 

 to such immediately beneficial results as the purification of city water 

 supplies at a trifling cost. 



DRY ROT OF THE POTATO. 



The wide-spread disease of the potato known as ' ' dry rot ' ' is caused 

 by a member of one of the lower orders of fungi, the Moniliales, Fusarium 

 oxysporum. This fungus has been known for nearly a century, but has 

 only recently been understood.* 



The potato plant is attacked through the roots, from which the under- 

 ground branches and tubers are invaded by the fungus. Toward maturity, 

 the affected plants are seen to be dwarfed and the root system is so weak- 

 ened that the stems become prostrated. Ultimately the foliage wilts, 

 dries, and turns dark brown. The tuber is occupied at its base by the 

 parasite, the presence of which is indicated by the discoloration of the 

 flesh. The damage from this disease is due chiefly to the spread of the 

 fungus in the tubers during storage. This may be checked b)' keeping 

 them cool and dry. Since the plants are infected from the soil, little 

 value is to be attached to spraying. At present there is no method known 

 for ridding the .soil of this fungous " weed." 



SEED DISPERSAL IN THE CASTOR-OIL PLANT. 



It has been calculated t that the amount of energy expended in pro- 

 jecting the seeds of one castor-oil plant of average stature and produc- 

 tiveness (bearing 696 seeds) is .0024 horse-powers per second. If each 

 plant requires 4 square feet of ground space, one acre would accommo- 

 date 10,890 plants. On this assumption, the amount of work by this 

 number of plants in exploding their carpels would be that capable of 

 being performed by " one good .strong horse "in 26 seconds. The cal- 

 culation is interesting, as the results indicate that there is an enormous 

 expenditure of energy by plants in ways that are generally but little 

 appreciated. 



The particular part that the plant plays in this matter is the produc- 

 tion of fruits with walls of such character that in drying the energy 

 required to expel the seeds is set free at once. The significance of this 

 to the plant is shown by the fact that, inasmuch as the seeds are thrown 

 to an average distance of 3.65 meters, in a period of 100 years 105 acres 

 would be populated by castor-oil plants, assuming, of course, environ- 

 mental conditions to be otherwise favorable. It should be pointed out 

 that this is only one method of dissemination. 



* Smith, E. F., and Swingle, D. B. " The Dry Rot of Potatoes Due to Fusarium oxysporum." 



tDandeno, J. B. "The Mechanics of Seed Dispersion in Ricinus communis." Bull. Torr. Club, 

 31 : 89. February, 1904. 



