GEOTROPISM. 



679 



and the short p-rowins; 



and perhaps nothing brings out tlie point here at issue so much as the fact that 

 the non-cellular plants behave, with reference to geotropism, exactly like those with 

 cellular structure, whence also every explanation of these phenomena which is 

 based on differences of cellular structure must be at once put aside as false. 



The fact here treated of generally may be expressed as follows. If the parts 

 of a plant are displaced by any cause whatever out of their original customary 

 position into a different one, they become curved until they again assume 

 the same inclination towards the horizon as before. This curvature, however, is 

 caused exclusively by growth, and hence only those organs which are still 

 capable of growth can regain their normal and original position with respect to the 

 horizon. If a plant grown in a flower-pot, for instance, is laid with its pot 

 horizontally, all those parts which are already completely developed retain the 

 new position, and only those which are still capable of growth commence after 

 some time to curve : the still growing shoot-axes, if they previously stood 

 upright, become curved until they again stand uprigh 

 portion of the primary root curves 

 until its apex is again directed vertically 

 downwards. Parts which originally 

 grew horizontally do not rest until they 

 have again become horizontal, those 

 usually oblique become curved until 

 they have resumed the same oblique 

 position. 



In order to simplify the de- 

 scription, I will first regard in what 

 follows those organs which originally 

 grow vertically upwards or downwards, 

 and assume that such organs have been 

 placed in a horizontal position. Fig. 

 383 will serve to illustrate what then 

 happens. ^ is a diagram of any seed- 

 ling whose plumule S originally grew vertically upwards, and its main root vertically 

 downwards ; this plant is now laid horizontally, care being taken that it can go on 

 growing. After a short time it is seen that the plumule has become curved upwards 

 as in ,5", until its apex is directed vertically upwards, and in like manner the primary 

 root has curved, at the short part which is growing in length, until it can again grow 

 vertically downwards. It is the custom to designate organs which behave like this 

 plumule as negatively geotropic, and those which behave like this primary root as 

 positively geotropic. The lines on m S mark a portion of the shoot-axis which is still 

 growing, and the negatively geotropic curvature may be especially observed here ; the 

 posterior transverse section u which lies at the limit of the fully developed basal 

 portion has not altered its position to any marked extent, the anterior transverse 

 section ou, on the contrary, has become displaced by the upward curvature into 

 the position o'u', and it is noticed that the under side of this portion of the shoot u u 

 has elongated considerably, whereas the upper side 00 has not elongated, or has 

 even become somewhat shorter. A similar stale of affairs is shown by the lines 



tlie shoot at fir: 



shoot of Crown Imperial {Frit- 

 I ; of the bulb s being cut away 

 ■, cr part of the scape d. The 

 .\\:\ after about twenty hours 

 cctcd itself through b into tlie 



