SPIRAL TWI2TER9. ' 



I bare just alluded to the twisting which necessarily follows 

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

 spire completed. This was well shown by painting straight lines 

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

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



I have already compared the revolving movement of a twining 

 plant to that of the tip 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 half 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 more slowly ; for 

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

 move slowly. If we 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 lie painted (this 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 the 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 Bapling Bpring up and bend it to the east, the black 

 line will then be seen on the lateral face (fronting 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 tho 

 southern lateral face; ami 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 bowed to the south: and let the lomri- 



