GROWTH AND MOVEMENT 467 



erect (figs. 1 195-1 197) . When the change of position can be shown to be 

 due wholly to gravity, this indicates that the peduncle undergoes with 

 age a change in its mode of response. Well-known examples are offered 

 by the snowdrop and the wind flower. Less generally known are like 

 changes in direction when certain stems, erect in the seedling stage, 

 develop into horizontal rhizomes in an older stage. 



Diageotropism. — Diageotropism of a somewhat modified type is seen 

 in the branches of the primary roots of some plants. These grow out 

 at a definite angle, and, if displaced, they will curve until the normal 

 angle is again attained. Similarly the oblique branches of trees some- 

 times are decidedly geotropic, and even the pendent ones may show it. 

 Only by the most cautious and precise experimentation in each case 

 can it be ascertained whether the positions assumed are due to gravity. 

 Unwarranted generalizations in this direction are particularly seductive. 

 In far the greater number of cases the position of organs is determined 

 by a complex of stimuli most difficult of analysis. 



Twiners. — Among the most interesting of the complex phenomena 

 are those exhibited by twining plants, in which geotropic reaction of a 

 peculiar kind plays a most important part. Twiners have slender stems 

 with a very long growing region, and a tardy development of the lateral 

 organs (leaves and branches), so that the long tips often look quite 

 naked. These ends seem to travel in a spiral fashion around some suit- 

 able, slender support, and the mature plant is thus wound around it 

 and clasps it tightly. At the outset the seedling, say of a morning glory, 

 grows quite erect, and seems like a parallelotropic plant, as, indeed, a 

 study of its reactions with a clinostat shows it to be at this period. After 

 reaching a certain height the tip no longer grows erect, but declines to 

 one side, and then a movement begins, quite like the irregular nutation 

 that every erect plant makes, except that it is regular and more striking. 

 The tip, standing in a nearly horizontal line, swings steadily around and 

 is directed successively to every point of the compass. This may bring 

 it into contact with a suitable support, around which it then proceeds to 

 twine, the free tip continuing the swinging movement from the point of 

 contact with the support. The fundamental feature of the twining, 

 therefore, is the swinging motion. 



Lateral geotropism. — Since the swinging movement does not con- 

 tinue when a twiner is properly rotated on a clinostat, it must be con- 

 sidered a response to gravity. As growth that can swing the tip sidewise 

 can be effective only if it takes place on the flank, the inference is made 



