4 INTEODUCTION. 



forms of circumnutation ; as again are the equally 

 prevalent movements of stems, &c., towards the zenith, 

 and of roots towards the centre of the earth. In 

 accordance with these conclusions, a considerable diffi- 

 culty in the way of evolution is in part removed, fv.ir 

 it might have been asked, how did all their diversified 

 movements for the most different purposes first arise ? 

 As the case stands, we know that there is always 

 movement in progress, and its amplitude, or direc- 

 tion, or both, have only to be modified for the good 

 of the plant in relation with internal or external 

 stimuli. 



Besides describing the several modified forms of 

 circumnutation, some other subjects will be discussed. 

 The two which have interested us most are, firstly, the 

 fact that with some seedling plants the uppermost 

 part alone is sensitive to light, and transmits an influ- 

 ence to the lower part, causing it to bend. If there- 

 fore the upper part be wholly protected from light, 

 the lower part may be exposed for hours to it, and yet 

 does not become in the least bent, although this would 

 have occurred quickly if the upper part had been 

 excited by light. Secondly, with the radicles of seed- 

 lings, the tip is sensitive to various stimuli, espe- 

 cially to very slight pressure, and, when thus excited, 

 transmits an influence to the upper part, causing it to 

 bend from the pressed side. On the other hand, if 

 the tip is subjected to the vapour of water proceeding 

 from one side, the upper part of the radicle bends 

 towards this side. Again it is the tip, as stated by 

 Ciesielski, though denied by others, which is sensitive 

 to the attraction of gravity, and by transmission causes 

 the adjoining parts of the radicle to bend towards the 

 centre of the earth. These several cases of the effects 

 of contact, other irritants, vapour, light, and tlie 



