POSITIVE AND NEGATIVE IIVDROTROnSM. 717 



have of penetrating into the soil, and of continuing their progress in it, does not 

 depend exclusively on their geotropism, but that the contact-stimulus and hydro- 

 tropism just described are also effective in llie matter; however, I cannot here go 

 more in detail into this subject. 



The klinostat experiment (Fig. 405) yields, however, yet another result. Apart 

 from disturbing influences, as at g, h, i, k, the plumules erect themselves so that they 

 stand out at right-angles on all the surfaces of the rotating cube, as a, c, d,/, for which I 

 can discover no other cause than this — that they likewise, but in an opposite way to 

 the roots, are hydrotropic — i. e. as the latter are so to speak attracted by a damp 

 surface, so die thin seedling stems are repelled by it : this is of course only to be 

 taken as a metaphorical mode of expression. The result is much more distinct, how- 

 ever, when, instead of a cube of peat, a cube of bread is fastened to the axis of the 

 klinostat and a few spores of P/iycimyccs, the Fungus we already know (p. 5, Fig. 3), are 

 sown on all its surfaces. The root-like mycelium penetrates into the bread antl there 

 becomes copiously branched, since it is not induced to grow downwards by 

 gravitation as in Fig. 394 : on the contrary, the sporangiophores (cf. Fig. 3, p. 5) 

 grow out from the bread, so that they all stand upright on the various surfaces of the 

 cube, as in Fig. 405 ?n m. If one of them comes out by chance at one of the 

 corners of the cube (as at m.^ it makes equal angles with the two neighbouring 

 surfaces. 



Wortmann, b}- means of sjiecial investigations with Phycomyces, has subsequently 

 confirmed my previous supposition that the sporangiophores are repelled by damp 

 surfaces, and I have no doubt that we may apply this result to the thin seedling 

 stems of Fig. 405 also. 



In concluding the consideration of the movements of plants induced by irri- 

 tability, I may again remind the reader of what I stated in the first introductory 

 lecture of this book, and what I have laid at the foundation of all considerations on 

 the vital phenomena of plants, that irritability is universal in the vegetable kingdom, 

 and that vegetable life without irritability is just as inconceivable as is animal life 

 without irritability. Irritability is the great distinguishing characteristic of living 

 organisms; the dead organism is dead simply because it has lost its irritability. 



