156 ANATOMY AND PHYSIOLOGY OF 



nished no result which admits of any essential addition to what 

 has been stated of Mimosa. The mtoving organ was always found 

 to consist of abundance of parenchymatous cellular tissue, which, 

 however, did not differ from ordinary cellular tissue in its visible 

 properties, and the contents of which, in like manner, presented 

 nothing characteristic, since they consisted of the usual substances, 

 starch, chlorophyll, fee, so that the conjecture is, indeed, not over 

 bold, that irritability may be a property belonging to cellular 

 tisKSue generally, which, however, can only display itself outwardly, 

 when developed in a higher degree, and where especially favour- 

 able conditions exist in the structure of an organ. The circum- 

 stance which is so distinctly marked in Mimosa^ that the side of 

 the sensitive organ, which becomes concave in the movement, is 

 alone capable of receiving the stimulus, wliile the opposite side is 

 perfectly insensible, appears to be universal ; at all events this 

 condition exists in like manner in the leaves of Dionoea, the 

 stamens of Berberis, and in tendrils. 



In the majority of cases, the movement of an irritable plant is 

 very transitory, when the plant is not organically injured by the 

 stimulus applied. Few expeiiments have been made on the 

 effects of long-continued irritation. An experiment of Desfon- 

 taines affords evidence that a plant may become accustomed to 

 a weak stimulus ; that author carried a Mimosa with him in a 

 coach, and in a short time the plant became accustomed to the 

 jolting motion, and its leaves, which at first had closed up, re- 

 opentd. It is otherwise with tendrils and twining plants, which, 

 when they come in contact with a foreign body, curl over the 

 point of contact, and in this way partially embrace the support. 

 A return to the former position is impossible, since through this 

 curvature a portion of the tendril or twining stem lying above 

 the point first izTitated is brought in contact with the support, 

 and in like manner stimulated to the curving movement ; in this 

 way the movement of curvature advances up the plant until the 

 whole length has become wound round the support. 



Ohse7'^. The view propounded by me (" Ueher den JBau und das Winden 

 der Rcmhen und Schlmgpfianzen'') that the curling round the support 

 results from an irritabihty excited by contact, hab not had to boaht of 

 any particular approval, yet 1 do not find that anything better has been 

 put in its place. "When Treviranns (" Fhf/sial" ii. 746) says that this 

 phenomenon is caused by a slowly and inertly acting elasticity, which is 

 chiefly called into activity by contact with foreign bodies, I must confe&s 

 that the meaning of these words is quite incomprehensible to me ; and 

 when Schleiden {'^Qfundzuge^' 2. ed. xi, 543) states it is a phenomenon of 

 growth, which determines the direction, that is, the pectdiar form of the 

 tendril, and the growth of the twining plant, he appears simply to be 

 ignorant of tbe fact which comes under consideration, since ^very accu- 

 rate exiuaiuation of tendrils and twining plants shews that the curlhig 

 round the suppoit is a phenomenon totally independent of the giowth. 



