510 PHYSIOLOGY OF GROWTH. 



When the pieces of Adoxa rhizome, placed with their basal ends 

 in soil, are unilaterally illuminated, from above or better from the 

 side, their growing apices in the course of a few days curve 

 vertically downwards. The downward curvature takes place some- 

 times towards one side, sometimes towards the other. It shows on 

 definite relations to the incidence of the light. The light here 

 also influences the geotropic behaviour of the organs in a char- 

 acteristic manner (heterogeneous induction of Noll). 



It is further instructive to make the following experiment, 

 which I carried out with shoots of Sida Napeea deprived of their 

 leaves, since it teaches that temporarily even geotropic after-effects 

 may be of importance in determining the direction of growth of 

 plant structures (see also 173). We stick the lower end of a 

 shoot of Sida into moist sand heaped against one wall of 

 a zinc box inside, and leave the shoot in a horizontal posi- 

 tion for 1J hours at a temperature of about 20 C., until a 

 geotropic upward curvature has just begun. We now rotate the 

 shoot 90 to the right or left. At the end of two or three hours 

 from this time, we find that the curvature of the shoot in the 

 horizontal direction has, owing to geotropic after-effect, become 

 very considerable. With this curvature, however, has been 

 combined another, an upward one, induced by the direct action of 

 gravity, so that the shoot curves obliquely upwards. 



It is of considerable interest to prove that, in some plant struc- 

 tures, the direction of growth is dependent, owing to their helio- 

 tropic peculiarities, on the position of the sun. If we examine 

 flowering specimens of Tragopogon orieiitalis growing in the open, 

 we shall find that the flower heads are only open in the morning. 

 They close in the course of the forenoon. In the hours of the 

 morning, the flower heads are directed towards the east ; during 

 the day they follow the course of the sun towards the south and 

 on to the west, as I often observed, and at night direct themselves 

 straight upwards. The movement of the heads of Tragopogon is 

 effected by the stem structure supporting them. At the time of 

 blooming this is in a state of intense heliotropic irritability, and 

 always grows more rapidly on the side which for the time being 

 is shaded than on that turned directly towards the sun, so that 

 the movements described must of necessity result. 2 



Radially constructed organs are very generally orthotropic; they 

 grow straight upwards or downwards. Plagiotropic organs, on 

 the other hand, are usually dorsi ventral. 



