33Q 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



a time and aerial rhizomes are produced. These experiments prove definitely 

 that tubers and rhizomes are really modified stems, in the sense of the plant 

 morphologists. • 



In the examples described above, of the experimental production of aerial 

 tubers and rhizomes, the nutrient materials, being unable to accumulate in 

 the usual subterranean storage organs, proceed to accumulate in the aerial 

 stems and so transform these 

 into storage organs. It is pos- 

 sible, however, to bring about 

 the accumulation of food ma- 

 terial in an entirely different 

 kind of organ from that in which 

 it usually occurs. For example, 

 in Boussingaultia baselloides , 

 which forms tubers under usual 



Fig. 169. — Swollen, tuber-like root, 

 developed at the cut end of the 

 petiole of a leaf of Boussingaultia 

 baselloides. 



Fig. 170. — Two segments of a willow twig, one 

 suspended in the normal (A) and the other in the in- 

 verted position (B). S, stem-pole; W, root-pole. 

 (After Vochting.) 



conditions, the accumulation of starch, etc., may be made to occur in the root. 

 To accomplish this, the petiole of a cut leaf is buried in soil. Roots develop at 

 the cut end of the petiole, and there results a simple kind of plant consisting of 

 a leaf and roots, without any stem. The organic materials produced in the leaf 

 accumulate in one of the roots in this case, which becomes greatly thickened 

 and forms a tuber-like storage root (Fig. 169). 



