DEVELOPMENT AND REPRODUCTION 



291 



the usual subterranean storage organs, begin 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 BoussingauUia 

 haselloides, which forms tubers under usual conditions, the accumulation of 

 starch, etc., may be made to occur 

 in the rpot. 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 



Fig. 169.— Swollen, tuber-like 

 root, developed at the cut end of the 

 petiole of a leaf of BoussingauUia 

 haselloides. 



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

 suspended in the normal (4) and the other in the in- 

 verted position (B). 5, stem-pole; W, root-pole. 

 {After Vochting.) 



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

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



Tn the physiological study of plant development and of the conditions con- 

 troUing this process, internal as well as external conditions must of course be 

 considered The existence of a polarity in stems, for example, was demonstrated 

 by Vochtingi in the foUowing manner. Cut pieces of a wUlow shoot are sus- 



. V8chting. Hermann, Ueber Organbildung im Pflanzenreioh. i and . Th. Bonn, rSyS and 1884: , 



