140 ANATOMY AND PHYSIOLOGY OF 



the mo&t diverse of their organs, which are sometimes dependent 

 on the influence of certain universally-diffused agents, such as 

 gravity and light, sometimes are excited by stimuli accidentally 

 affecting them, and sometimes occur independently without the 

 existence of any demonstrable external cause. Great as the simi- 

 larity to animal motion is in many of these movements, they are 

 always devoid of the character of volition, so that altogether no 

 more definite and profound distinction between plants and ani- 

 mals can be found, than the total want of voluntary motion in 

 the former and the presence of this same in the latter. 



Ohserv. Unfortunately it is extremely difficult to make out, in many 

 cases, whether a motion is s^oluntary or not, yet repeated unprejudiced 

 observations will very rarely leave a doubt about it. In no other inves- 

 tigation does the obseryer need calm reflection in so high a degree as 

 here, for hundreds of examples shew how readily the imagination steps 

 in and leads to erroneous conclusions in the observation of the enigma 

 tical movements of plants. "Warning examples are furnished by ob- 

 servations on the '^swai'ming" spores of Alg^, on the Diatomaceae, 

 Oscillatoriese, (fee, in abundance ; shewing how soon, when once the kiad 

 of motion has been mistaken and these plants conceived to be animals, 

 their entire structure has become misunderstood, and imaginary eyes, 

 intestines, feet, and other animal organs have been discovered, which 

 more temperate observers have recognrzed as things differing as widely as 

 the poles. 



In examining the movements of plants we must first of all 

 exclude those cases in which motion of an organ is caused by the 

 more or less complete drying up of different layers of it, producing 

 unequal contraction and thereby curling of the parts. The rapidity 

 of the motion produced in this way depends on the mechanical 

 conditions of structure ; it may be slow or very rapid. The former 

 is the case when no external hinderance opposes the movement of 

 the drying organ, the latter when the curving part is blended with 

 other parts, so as to be hindered from following its contraction, 

 whereby a gradually increasing tension arises in it, the final re- 

 sult of which is a rupture and a sudden relief of the stretched 

 part, like the recoil of a metal spring. In general, the layers of 

 an organ which contract most strongly in drying are those which 

 are composed of larger, thinner-walled, and more globular cells, 

 while a layer composed of thick-walled, small, and elongated 

 cells suffers less contraction, and therefore forms the convex side 

 of the curved organ. 



Ohsm^v. Examples of these hygroscopic movements are of every-day 

 occurrence, and it will suffice to indicate a few of them. Among these 

 are, —the contraction into a globe, which the ramified stems of many 

 plants, such as Anmtatica MeTOchontica, Lycopodiwm lepidophyllum, im- 

 dergo in drying ] the dehiscence of anthers, the burstiag of most dry 

 fruit, the rupture of the outer seed-coat of Oxalis, the twisting of the 

 awns of many grasses. In particular cases even isolated pieces of cell- 

 waU. exhibit movements of this kind, when their various layers differ 



