SPIRAIi TWIKERS. 7 



I have jast alluded to the twisting which uecessarily follows 

 from the spiral ascent of tlie stem, namely, one twist for each 

 spire completed. This was well shown hy painting straight lines 

 on stems, and then allowing them to twine ; but, us I shall have 

 to recur to this subject under Tendrils, it may be here passed over. 



I liare already compared the revolving movement of a twining 

 plant to that of the tii> of a sapling, moved round and round by 

 the hand held some way down the stem ; but there is a most im- 

 portant difference. The upper part of the sapling moves as a 



rigid body, and remains straight ; but with twining plants every 

 inch of the revolving shoot has its own separate and independent 

 movement. This is easily proved ; for when the lower Jialf or 

 two-thirds of a long revolving shoot is quietly tied to a stick, the 

 upper free part steadily continues revolving : even if the whole 

 shoot, except the terminal tip of an inch or two in length, be tied 



up, this tip, as I have seen in the case of the Hop, Ceropegia, 

 Convolvulus, &c., goes on revolving, but much luore slowly; for 

 the internodes, until they have grown to some little length, always 

 move slowly. If wo look to the one, two, or several internodes 

 of a revolving shoot, they will be all seen to be more or less 

 bowed either during the whole or during a large part of each 

 revolution. Now if a coloured streak be painted (tliis was done 

 with a large number of twining plants) along, we will say, the 

 convex line of surface, this coloured streak will after a time (de- 

 pending on the rate of revolution) be found to lie along one side 

 of the bow, then along the concave side, then on the opposite side, 

 and, lastly, again on tlie original convex surface. This clearly 

 proves that the internodes, during the revolving movement, be- 

 come bowed in every direction. The movement is, in fact, a con- 

 tinuous self-bowing of the whole shoot, successively directed to 



all points of the compass. 



As this movement is rather difficult to understand, it will be 



well to give an illustration. Let us take the tip of a sapling and 



bend it to the south, and paint a black line on the convex surface ; 



then let the sapling spring up and bend it to the east, the black 



line will then be seen on the lateral face (fi'onting the north) of 



the shoot ; bend it to the north, the black line will be on the 



concave surface; bend it to the west, the line will be on the 



southern lateral face ; and when again bent to the south, the line 



will again be on the original convex surface. Now, instead of 



bending the sapling, let us suppose that the cells on its whole 



southern surface were to contract from the base to the tip, the 



whole shoot would be bQWfd ^p tl^e aouthj and let the lopgi- 



