728 ECOLOGY 



as in the maple (fig. 779), doubtless are due to the same cause as are 

 variations in the length of stems, that is, to differential shading or more 

 probably to moisture differences due to differential shading. For a con- 

 sideration of the striking elongation of pendulous stems, see p. 657. 



Elongation in aquatic stems and petioles. The phenomena. In 

 water plants with floating leaves, (as in Polygonum amphibium) or with 

 emersed leaves (as in Hippuris), the length of the stem may vary widely, 

 depending upon the depth of the water. In swamps that have been 

 flooded, stem elongation may be extraordinary, ten times the usual 

 length having been reported; for example, Eleocharis stems, usually but 

 a few centimeters in length, have been known to grow to a length of a 

 meter and a half. Differences in the length of aquatic petioles are more 

 common, and if anything more striking. The petioles of the floating 

 leaves of the water lilies, Castalia and Nymphaea, vary with the depth of 

 the water in which they grow, their length commonly being somewhat 

 greater than the depth of the water, a phenomenon that results in oblique 

 orientation. In emersed leaves (as in Sagittaria) the petiole length varies 

 similarly, and under experiment the leaves continue to emerge, even if 

 the depth of the water is greatly increased. Elongated water stems and 

 petioles, like elongated aerial organs, are very slender, so that an in- 

 creased amount of structural material is not necessarily involved in elon- 

 gation. As might be expected, it is possible experimentally to find a 

 depth too great for the elongating petiole to raise its blade into the air. 



The probable factors. The striking fact in hydrophytes with floating 

 leaves is not so much the varying length of stems and petioles as the 

 cessation of growth at the water line, as though at that level some new 

 factor came suddenly into activity. Such a factor, of course, is transpi- 

 ration, and it seems probable that this is the chief factor involved in such 

 cases, since growth in covered aquaria, where transpiration is greatly 

 reduced, results in the development of emersed leaves in place of floating 

 leaves in Hydrocharis, Nymphaea, and Potamogeton natans. Short 

 aerial stems and petioles as compared with the corresponding elongated 

 aquatic organs also would seem adequately accounted for by differences 

 in transpiration. Variations in oxygen content sometimes have been 

 thought to explain differential elongation in aquatic stems, though it is 

 not easy to see why a stem should grow longest where the oxygen supply 

 is smallest, nor why there should be a sudden arrest of growth at the 

 water line. Similarly, differences in light intensity often are cited as 

 factors in the elongation of aquatic stems as well as of aerial stems. 



