STEMS 



729 



While diminished light might cause elongation in deep water, the in- 

 crease of light in passing from water to air is not sufficiently sudden to 

 account for the arrest of growth at the water line. The failure of deeply 

 submersed leaves to reach the surface may be due to diminished synthe- 

 sis, and thus indirectly to light. The mechanical support given by 

 water occasionally has been thought to have some connection with 

 the great elongation in that medium. 



Elongation in stems submerged by sand. — The phenomena. — Still more re- 

 markable than the elongation of aquatic stems is the elongation exhibited by certain 



1047 



1048 



1049 



Figs. 1047-1049. — Diagrams showing the stem elongation of certain plants (as wil- 

 lows or dogwoods) when submerged by advancing sand dunes: 1047, a dune in whose 

 path of advance is a swamp with such shrubs of ordinary height; 1048, the same place a 

 few years later, the dune having advanced; note that the stems of the shrubs have twice 

 their usual length; 1049, the same place, after the lapse of a few more years; the stems 

 of the shrubs have four times their usual length. 



aerial stems that are partially buried by the sand of moving dunes. While some 

 trees (as oaks and pines) soon succumb to the sand, others (as the white elm and the 

 red maple) remain alive for years, unless completely covered. A number of trees 

 and shrubs, notably willows (as Salix syrticola and 5. glaucophylla), dogwoods 

 (as Cornus stolonifera), and poplars (as Populus deltoides), not only remain alive, 

 but are stimulated to a growth far exceeding that of ordinary aerial stems. Dog- 

 woods and willows, which in ordinary habitats rarely attain a height of more than 

 two or three meters, may have stems twelve, fifteen, or even eighteen meters above 

 the original ground level, if partially submerged by sand (figs. 1047-1049). The 

 growing stems usually keep pace with the rising sand, so that the height above the 

 ground remains about the same year after year, approximately equalling the usual 

 height of the shrubs. 



