STEMS 



669 



may aid in determining the position of rhizomes. Furthermore, be- 

 havior may vary with the season, stems often appearing progeotropic or 

 diageotropic in the' autumn (fig. 981) and apogeotropic in the spring 

 (figs. 979, 980). Experiments have shown that many shoots are diageo- 

 tropic at low temperatures and apogeotropic at high temperatures. 

 Obviously, progeotropic and diageotropic reactions result in increased 



FIGS. 979, 980. Rhizomes of the cord grass (Spartina cynosur aides): 979, the basal 

 portion of a shoot, as seen in summer, showing the origination of new rhizomes (r~), which 

 are completely ensheathed by overlapping scale leaves (s) ; 980, a similar shoot, as seen early 

 in the following spring; note the sharp change in the growth direction of the rhizomes 

 (/') from progeotropic to apogeotropic. 



protection from transpiration and from cold. In Circaea and in Sola- 

 tium titbcrosum, the rhizome develops into an erect shoot, when the aerial 

 stem is removed. This experiment appears to show that such rhizomes 

 are apogeotropic organs whose customary horizontality is due to the 

 inhibition of verticality by the erect stem, much as the growth direc- 

 tions of lateral stems and roots are due to the inhibition of verticality 

 by the primary organs. 



Linear and radial migration. In the simplest rhizomes, branching is 

 comparatively slight, and each season there is a new growth of several 



