342 



BOTANY 



PART 



can also be stimulated by 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 

 placed horizontally the growth is limited to the lower side while the upper side is 

 passively compressed (Fig. 279). 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, 

 and in the leaf-sheaths of the cotyledons of some Liliaceae and in the 

 rhizome of Yucca. All these organs, 

 when placed in any other position, 

 assume a straight downward direc- 

 tion and afterwards maintain it. 

 Positively geotropic, like negatively 

 geotropic, movements are possible 



FIG. 279. Geotropic erection of a grass-haulm 

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

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

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

 lengthened, the upper side (o) somewhat 

 shortened ; as a result of the curvature the 

 grass-haulm has been raised through an 

 angle of 75. (After NOLL.) 



FIG. 280. Geotropic curvature of the roots 

 of a seedling of Vicia Fdba. I, Placed 

 horizontally ; 77, after seven hours ; 177, 

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

 (After SACHS.) 



only through growth. The power of a downward curving root-tip 

 to penetrate mercury (specifically much the heavier), and to overcome 

 the resistant pressure, much greater than its own weight, proves 

 conclusively that positive geotropism is a manifestation of an active 

 process. Positive geotropic curvature is due to the fact that THE 



GROWTH OF AN ORGAN IN LENGTH IS PROMOTED ON THE UPPER 

 SIDE, AND RETARDED ON THE SIDE TURNED TOWARDS THE EARTH. 



Fig. 280 represents the course of the geotropic curvature in a root. 



Most lateral branches and roots of the first order are plagiogeotropic, while 

 branches and roots of a higher order stand out from their parent organ in all direc- 

 tions. THESE ORGANS ARE ONLY IN A POSITION OF EQUILIBRIUM WHEN THEIR 



LONGITUDINAL AXES FORM A DEFINITE ANGLE WITH THE LINE OF THE ACTION OF 



GRAVITY. If forced from their normal inclination they return to it by curving. 

 A special instance of plagiogeotropism is exhibited by strictly horizontal organs, such 

 as rhizomes and stolons, which, once they have attained their proper depth, show a 

 strictly TRANSVERSE GEOTROPISM (diageotropism). Should the proper depth not be 



