328 



PHYSIOLOGY OF GROWTH AND CONFIGUE ATION 



autumn, when they are full-grown and are about to die, growth is resumed after 

 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 possible, however, to 

 bring about the accumulation of food material in an entirely different kind of 

 organ from that in which it usually occurs. For example, in Boas sin gaultia 

 baselloides, which forms tubers under usual 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 con- 

 sisting of a leaf and roots, with- 

 out any stem. The organic 

 materials produced in the leaf 



Pig. 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 Yachting.) 



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

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



