458 LECTURE XVIII. 



germinating seeds are made to descend, and their plumules to 

 ascend, by some external cause, and not by any power in- 

 herent in vegetable life ; and I see little reason to doubt that 

 gravitation is the principal, if not the only agent employed in 

 this case by Nature." 



The downward direction of growth of orthotropic roots 

 and the upward direction of growth of orthotropic stems are 

 due, then, to the action of gravity ; the direction of growth is, 

 in fact, the response of the growing organ to the stimulating 

 effect of gravity, just as the assumption of a definite direction 

 of growth under the influence of light is the response of the 

 growing organs to its stimulating action. Further, just as we 

 found organs which curve towards or away from the source of 

 light, organs which are respectively said to be positively or 

 negatively heliotropic, so we find organs which grow towards 

 or away from the centre of the earth, and they are respectively 

 said to be positively or negatively geotropic, or, to use Darwin's 

 terminology, geotropic or apogeotropic. 



We have already cited primary stems as examples of 

 negatively geotropic shoots, and primary roots as examples of 

 positively geotropic roots ; but to these many more may be 

 added. The stipes of Mushrooms, the sporangiferous hyphae 

 of Moulds, the stems of Characeae, the stalks of the receptacles 

 of Liverworts, the seta of the Muscineae, the peduncles of many 

 flowers, are examples of negatively geotropic shoots : the 

 hyphae of Moulds which penetrate into the substratum, the 

 root-like filaments of Vaucheria, Caulerpa, and other Algae, 

 the rhizoids of Muscineae, are examples of positively geotropic 

 roots. The long narrow radial leaves of some Monocotyledons, 

 such as the Onion, are further examples of negatively geotropic 

 organs. Orthotropic shoots are generally negatively geotropic, 

 but not in all cases, for Sachs points out that the rhizomes of 

 Yucca filamentosa and of Cordyline rubra grow downwards like 

 tap-roots. Nor are all orthotropic roots positively geotropic, 

 for, as Schimper has found, the climbing roots of various 

 Epiphytes are negatively geotropic. 



Cases of the absence of geotropic irritability are afforded 

 by the hypocotyl of the Mistletoe (Viscum) which, as Duhamcl 



