ii4 THE WATERLILIES. 



I know of no experiments upon the geotropism of stems outside of 

 the Eu-castalia group. Among these plagiotropism is very pronounced. 

 A stem of N. odorata 3 cm. in diameter and 10 cm. long planted upright 

 in a pot in early spring had turned its apex to a horizontal position by 

 mid-summer. 



Young petioles and peduncles of different species turn through 90 

 in the course of twenty-four hours if placed in a horizontal position. This 

 tendency is, of course, well developed in all species. The leaf-stalks and 

 flower-stalks, however, are often so slender that their positions are mostly 

 passive, except at the ends. The buds in Castalia, and the leaves in all 

 species, by their buoyancy make their way to the surface of the water 

 unless opposed by other forces. We have already noted that the leaf in 

 coming up through the water has its midrib in a straight line with the 

 petiole, and on reaching the air the petiole bends over at the top nearly 

 at right angles to the lamina (Frank, 1872). That this is not a purely 

 passive condition is shown by the stiffness and turgidity of the organs. 

 Also, when plants are moved close together or placed in small tanks, 

 where the upper parts of the petioles are thrown out of their former 

 directions, they bend the lamina down until it is more or less submerged. 

 In the course of twenty-four hours, however, the leaf is again lying flat 

 upon the water surface. A corresponding bending has taken place in the 

 upper part of the petiole. This bending may be due simply to readjust- 

 ments of a mechanical kind in the elastic collenchyma of the petiole, or it 

 may be the result of a distinctly vital action. In the latter case it is related 

 to the phenomena next to be described. 



It has been noted that the " length of the petioles and peduncles 

 depends upon the depth of the water in which the plant is growing." 

 Not only is this true for plants in different localities, but a given plant, 

 if moved from shallow into deeper water, where all of its leaves are sub- 

 merged a few centimeters, will be found on the following day with all 

 its leaves floating in the normal manner. Plants moved from deep to 

 shallow water need no adjustment except in the angle between the petiole 

 and lamina ; but leaves produced afterward have petioles only long enough 

 for the new conditions. Now, it is certain that throughout the life of a leaf 

 its petiole is continually growing longer, even though the water level 

 remains constant. This is necessary in order to make room for the new 

 leaves which are rising in succession from the center of the plant. One 

 need only look at a large plant of any tropical Nymphaea as grown in 

 gardens to be convinced of this. We see an area ten or twelve feet in 



