GROWTH OF CREEPING STEMS. 103 



most commonly the tree is destroyed, ere long, by the insufficient 

 supply of nourishment to the roots. 



145. Now such obstructions not unfrequently arise from 

 natural causes. There are several creeping plants, whose habit 

 it is closely to embrace the stems round which they coil ; such is 

 the common Bindweed of this country ; and in tropical climates., 

 these creepers are more numerous, and their stems more woody. 

 These seldom wind in complete rings, but in a spiral, growing 

 like a corkscrew; and thus the descent of the sap is rather 

 obstructed than prevented. But an accumulation of the nutri- 

 trious fluid takes place above the whole line of the spiral : so 

 that, when the creeper is removed, the stem presents the curious 

 appearance of a deep indentation, passing round it from one end 

 to the other; and on the upper edge of this, a corresponding 

 elevation. Endogens are subject to no such alteration ; as the 

 sap does not pass down the exterior of the stem, and its diameter 

 increases but little. 



146. Such creepers are exceptions to the general rule, that it 

 is the tendency of stems to grow vertically or right upwards. 

 This tendency is sometimes shown in a very curious manner. 

 If the trunk of a young tree be artificially bent, by drawing it 

 (for instance) by a cord to one side, the branch which then most 

 nearly approaches the vertical direction, will increase more than 

 the rest, and will at last appear quite continuous with the lower 

 part of the stem. Again, if the trunk of a tree which usually 

 throws out its branches almost horizontally, such as the Elm, be 

 broken off, the highest branches will gradually approach the 

 upright position, so as to appear like continuations of the broken 

 trunk. In coiling stems, however, it would appear as if some 

 tendency to turn to one side was constantly operating, in con- 

 function with the upright growth ; so that a cork-screw-like 

 form is produced. 



147. It is a little remarkable that, though this turn is usually 

 in the contrary direction to that in which the sun appears to 

 move, (as is the case in the common Bindweed, most plants of 

 the Pea tribe, the Passion-flower, the Dodder, and many others,) 

 it is sometimes the same with it, as in the Hop (Fig. 54). Almost 



