2] THIGMOTROPISM 377 



ists. A study of the cause of this form was made by PALM 

 ('27), by MOHL ('27), and by DUTROCHET ('43, '44) ; and 

 from this early period to the present there have existed on this 

 matter great differences of opinion. On the one hand, there 

 has been maintained a view that had great inherent probability, 

 the view, namely, that the twining of plants is a response to 

 the contact-stimulus of the object about which they are coiled. 

 Against the general validity of this view certain experiments 

 of DARWIN ('82, p. 16) seem conclusive. For he found that 

 even the hard rubbing of the stalk caused no modification of 

 the normal spiral growth. Accordingly, the conclusion seems 

 generally accepted to-day that the peculiar form of growth of 

 most twining plants is the combined result of geotropism, by 

 which the stem grows upwards, and a special form of nutation, 

 by which it impinges against the supporting stick and bends 

 round it. The twining is thus mechanical, depending upon 

 the structure of the stem, -- rather than responsive. 



An exception among twining plants is found in the dodder, 

 Cuscuta. This plant is a parasite, belonging to the Convol- 

 vulus family, and lives upon the flax and other plants. It is 

 leafless, and twines closely about its host, into which it sends 

 the feeding organs --the haustoria. According to the careful 

 studies of PEIRCE ('94), the stem, when not in contact with 

 any solid body, makes long, steep turns about the axis of the 

 spiral, as is the case with other twiners. If now the free stem, 

 during a period of slow growth, be brought into contact with a 

 wooden or glass rod or a thread at about 3 cm. from the tip, the 

 stem bends sharply, and in the course of 15 hours makes two 

 or three close turns around the vertical support. If the con- 

 tact be made at a distance of only about 1 cm. below the tip, 

 there will be little or no change in the character of the twining. 

 If the point of contact be too far below the tip, 6 to 7 cm.,- 

 there will also be no effect. The result is thus clearly depend- 

 ent upon a stimulus applied at a definite sensitive point ; it is 

 a typical response. 



2. Tendrils. - - Very similar to the phenomena of close twin- 

 ing in Cuscuta is that of coiling in the tendrils of phanero- 

 gams those most marvellously sensitive of all plant struct- 

 ures. DARWIN ('82) and PFEFFER ('85), particularly, have 



