STEMS 745 



induced by various substances which check the growth of shoots. An 

 instance of tuberized aerial stems is seen in the kohl-rabi ; even the leaves 

 of this plant become tuberized if flowering is suppressed. Similar tuber- 

 ous swellings occur on the stems of Eucalyptus; neither in the kohl-rabi 

 nor in Eucalyptus are the stimulating factors known except that in the 

 former, light is necessary, and that in the latter, the size increases with 

 the food supply. In Nephrolepis similar primordia may, as the con- 

 ditions vary, give rise to rhizomes, runners, or tubers. 



The fungal theory of tuberization. It has long been known that tubers 

 almost universally are infected with fungi, potato tubers, for example, 

 containing Fusarium Solani and other fungal forms. The experimental 

 study of the tuber problem as related to fungi is attended with difficulty, 

 owing to the fact that the isolation and the subsequent cultivation of 

 tuber fungi, as of mycorhiza fungi in general, is far from easy. As yet, 

 inoculation experiments with potato rhizomes are uncertain in their 

 results, but in many orchids, where fungal symbiosis is more obligate 

 than elsewhere, some exceedingly suggestive studies have been made. 

 As a class, orchids have two growth periods, one of relatively active dif- 

 ferentiation and elongation, and another of lessened activity and dia- 

 metral increase, that is, of tuber formation. Tuberization may involve 

 the roots, as in Habenaria, the underground stem, as in A plectrum, or 

 the aerial stem, as in most epiphytic forms. It has been shown by inoc- 

 ulation experiments and otherwise that tuberization is initiated when the 

 stem or root is infected with the proper fungus, the latter organism ap- 

 pearing to arrest the growth of the terminal bud and to cause the develop- 

 ment of hypertrophied cells. Shoots arising from infected tubers are at 

 first free from fungi, and they differentiate rapidly until they in turn are 

 infected, whereupon tuberization sets in, so that usually there is one 

 period of infection and tuberization each year in a given rhizome or root. 

 In the shoots of Neottia, however, fungal infection and tuberization are 

 present from the start. Simultaneously with tuberization starch accumu- 

 lation is greatly accelerated. 



Circumstantial evidence favoring the fungus theory. When the potato was first 

 introduced into France, gardens sown to tubers produced crops of tubers, while 

 gardens sown to seed produced no tubers, the inference being that the fungi from 

 the old tubers infected the new tubers, whereas the seeds were free from fungi. 

 Nowadays seed cultures produce crops of tubers, presumably because the soil has 

 by this time become infected with the necessary fungus. The gametophytes of 

 Lycopodium and Botrychium are tuberous and are infected by fungi when subter- 



