310 



BOTANY 



PART I 



erection of a seedling growing in semi-darkness and placed in a horizontal position 

 (No. 1). The growth in the stem of the seedling is strongest just below the cotyledons, 

 and gradually decreases towards the base. The curvature begins accordingly close 

 to the cotyledons, and proceeds gradually down the stem until it reaches the lower, no 

 longer elongating, portions. Through the downward movement of the curvature, and 

 partly also owing to the after-effect of the original stimulus, the apical extremity 

 becomes bent out of the perpendicular (No. 7), and in this way a curvature in the 

 opposite direction takes place. For two reasons this excessive curvature must 

 again diminish. In the first place ever}' curved part of a plant, whatever the 

 cause of the curvature has been, tends to straighten (autotropism). This is shown 

 when a geotropically curved shoot is rotated on a klinostat. This autotropic 

 tendency is naturally only effective when geotropism does not oppose it. In the 

 case in question, however, after the 



excessive curvature another geotropic ojtz^_hA- 



stimulus in the opposite direction to ^-— 



the first combines with the autotrop- * 



ism. The final result is thus ahvays 

 sooner or later a complete straightening 

 of the stem. 



Fig. 231. — Geotropic erection of a grass-haulm 

 by the curvature of a node. 1 , Placed hori- 

 zontally, both sides («, o) of the node being 

 of equal length ; 2, the under side (w) 

 lengthened, the upper side (o) somewhat 

 shortened ; as a result of the curvature the 

 grass-hauliu has been raised through an 

 angle of 75°. 



jr 



m 



Flo. 232. — Geotropic curvature of the roots 

 of a seedling of Vina Fabc I, Placed 

 horizontally ; 11, after seven hours ; 111, 

 after twenty-three hours ; Z, a fixed index. 

 (After Sachs.) 



In some cases negatively geotropic curvatures may take place in full-grown 

 shoots, i.e. in such as no longer exhibit growth in length when not geotropically 

 stimulated. Thus in woody stems and branches the growth in length of the 

 cambium of the lower side may bring the organ into the erect position as a result 

 of geotropism. The greater the resistance of the parts which have to be passively 

 bent the more slow and incomplete will this response be. The so-called nodes of 

 grasses Avhich in reality are leaf-cushions, can also be stimulated bj^ geotropism to 

 further growth. If the stimulus acts on all sides, as when the node is horizontally 

 placed and rotated on the klinostat, all the parenchymatous cells exhibit a uniform 

 elongation. If the node is simply jilaced horizontally the growth is limited to the 

 lower side while the upper side is passively compressed (Fig. 231). By means of 

 such curvatures in one or several nodes grass haulms laid by the wind and rain 

 are again brought into the erect position. 



Positive geotropism is exhibited in tap-roots, in many aerial roots, ami in the 

 leaf-sheiths of the cotyledons of some Liliaceae and in the rhizome of Yucca. All 

 these organs, when jjlaced in any other position, assume a straight downward 



