THE VEGETABLE CELL. 143 



produced by not less than three causes (the influence of gravity, 

 light, and contact of vsolid bodies). Since, however, the external 

 influences upon which these movements depend, are as yet alto- 

 gether unexamined, and thus nothing but empty guesses can be 

 expressed regarding them, I feel that I ought to be content 

 with the indication, that even in the simple cell, phenomena of 

 motion occur which are comparable with those of the compound 

 organs. 



With regard to the motions of the parts of higher plants, so far 

 as they are" connected with their growth, we are first struck by 

 the determinate directions of the root, the stem, and the leaves. 

 To no phenomenon have we been rendered — by having it daily 

 before our eyes — more indififerent, than to the definite direction 

 in which every part of a plant lies in reference to a perpendicular 

 line ; and yet in the circumstance that the stem grows upwards, 

 the root downwards, and the leaf with its upper surface turned 

 towards the sky, we behold a series of the most wonderful pheno- 

 mena, the causal relations of which are unfortunately but too 

 little understood. These positions of the various organs seem 

 to us so natural, that it is only by the exceptions which occur in 

 many plants, and by the effort of a dis-arranged part to regain 

 its normal position, that our attention becomes attracted to the 

 fact that this position is the result of a series of mysterious pro- 

 cesses, which, though unnoticed, are ever active in the plant. 



Experiments of the simplest kind, in particular such as were 

 made by Duhamel, have long since shewn that the earlier endea- 

 vours to explain the gi'owth of the root downwards and of the 

 stem upwards, from the influence of the darkness and moisture 

 of the soil upon the root, and the brightness and dryness of the 

 air upon the stem, were mistaken, for under all circumstances, be 

 the position and the surrounding media of a germinating seed 

 what they may, whether this germinate in the earth, in air or in 

 water, in darkness or under the influence of light, the radicle and 

 the plumule will curve until they have acquired their normal 

 direction. The acuteness of Knight ("PMZ. Trans!' 1806; "-4. 

 Selection from the Physiolog. Papers," 124) obtained the first 

 success in demonstrating sure evidence of the connexion of this 

 phenomenon with the effect of a determinate force, of gravity, in 

 fact, by making seeds germinate on wheels rotating rapidly, under 

 which circumstances the radicles turned towards the p^^iphery, 

 and the plumules towards the centre of the wheels. This experi- 

 ment was afterwards extended by Dutrochet to the leaves, where- 

 by he shewed that the leaves are also subject to the eflfect of 

 gravity, for these turned their lower faces towards the peiiphery 

 of the wheel (Dutrochet " Memoires,"' ii. 54). 



Oherv. It is difficiilt to conceive how any naturalists could question 

 tlie vahxe of the evidence furnished by this experiment, in which the 



