570 BOTANY OK CROP PLANTS 



When the potato was first introduced in France, it was 

 found that when tubers were planted a crop was produced, 

 but when seed was sown no tubers were obtained. From 

 this it was inferred that when tubers were planted they in- 

 fected the new ones, while the seed, free of fungi, did not 

 furnish a supply to infect the stolons, and hence tubers could 

 not form. However, no difficulty is now experienced in se- 

 curing tubers from seed because the soil has Become inocu- 

 lated with the proper fungi. If this theory is correct, and 

 there seems to be some evidence that it is, the potato tuber 

 is in reality a gall, produced by a foreign organism. 



In the potato, tuberization has been induced in concen- 

 trated solutions of sucrose or glycerin, etc., independent of 

 fungi. Similar results have occurred in the case of orchids, 

 onions, and radishes. From this it seems that the formation 

 of tubers may result when the osmotic pressure in the cul- 

 tural medium is high. However, this alone does not appear 

 to be the only determiner, since different results follow the 

 use of glucose and glycerine solutions of equal pressure. It 

 certainly seems that plenty of sugar must be present for 

 starch formation, and perhaps also for tubers to form. From 

 this fact, that a more concentrated cell sap is usually present 

 in fungi than m other plants, it does not seem unreasonable 

 to suppose that the role of fungi in tuberization consists in 

 raising the concentration of the media which they enter. It 

 has actually been found that cultures of Fusarium in macer- 

 ated potato tuber preparations increase the concentration. 

 In this connection, it is suggested that low temperatures and 

 dryness of soil may induce tuberization through increasing 

 the concentration of cell sap. 



History. — It seems "that the potato was cultivated and 

 utilized by the Chilean and Peruvian people before the 

 arrival of the Spaniards. In 1533, Pizarro found the Chileans 



