SOLANACE^ 569 



developed on a plant grown from seed, come at the tips of 

 stolons arising on. the stem above ground. However, when 

 the tuber as a cutting is used in propagation, the young 

 tubers form at the ends of long, thin rhizomes (undergroimd 

 stems) which arise underneath the ground from the main 

 axis or stem (Fig. 12). The length of the stolon seems to 

 be constant and a strong variety characteristic. In culti- 

 vated varieties, it should not exceed 3 or 4 inches. In 5. 

 commersonii, it is reported as sometimes reaching a length 

 of 10 feet. The tubers or swollen stems bear a number of 

 buds, and these buds send out sprouts when the tuber is 

 planted. 



As a rule, the tubers are formed beneath the ground as 

 noted above; but in abnormal cases, or when disturbed by 

 diseases, the above-ground stems may produce tubers. For 

 example, when the fungus, Rhizoctonia, which shuts off the 

 downward movement of elaborated foods from the leaves to 

 the underground tuber-forming stems, is active, normal 

 tuberization under ground is interfered with and the stems 

 above ground will have a tendency to swell and produce 

 small tubers. This phenomenon is often indicative of 

 Rhizoctonia. 



Fungus Theory of Tuberization. — In general, it is found 

 that darkness and low temperature favor the development 

 of potato tubers. Tuberization is also facilitated somewhat 

 by checking the growth of shoots or fruit. 



There is some basis for the theory that the formation of 

 the tubers is associated with the presence of certain fungi. 

 It is certain that tuber production is encouraged in certain 

 orchids when the stem or root is infected with the proper 

 fungus. The fungus appears to check the growth of the 

 terminalbud and cause the development of hypertrophied 

 cells. 



