GROWTH AND MOVEMENT 467 
erect (figs. 1195-1197) . 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 
