THE STEM 



53 



stems of other plants for support (Fig. 38), and often 

 grow until they spread their own leaves above those of the 

 plants that they encum- 

 ber. The wav in which 

 such climbers bend from 

 side to side until they 

 strike some vertical sup- 

 port may be told in the 

 words of Darwin : — 



38. Twining stem of the 

 Morning Glory. 



" When the shoot of a hop 



rises from the ground, the two 



or three first-formed joints or 



internodes are straight and 



remain stationary; but the next-formed, whilst very young, may be 



seen to bend to one side and to 4;ravel slowly aronijd towards all 



points of the compass, moving like the hands of a clock, with the sun. 



The movement very soon acquires its full ordinary velocity. From 



seven observations made during August, and on another plant during 



April, the average rate during hot weather and during the day is two 



hours eight minutes for each revolution ; and none of the revolutions 



varied much from this rate. The revolving movement continues as 



long as the plant contiimes to grow ; but each separate internode, 



as it becomes old, ceases to move." 



74. The revolutions are less rapid at night than in the 

 daytime, but are maintained until some object of support 

 is met with, when the free extremity still goes on revolv- 

 ing and the stem shortly encircles the support. The 

 movement then continues in an upward-winding spiral, 

 the coils tightening and the twiner steadily ascending. 



75. Most species of twining plants wind in a definite 

 direction. That is, as we look down upon the plant, the 

 revolving tip moves with the hands of a watch lying face 

 upward, in some species ; opposite to the hands, in other 

 species. 



76. Another class of climbing plants includes those that 

 simply clamber in a haphazard fashion through and over 

 the surrounding herbage. The thorns of many Brambles 

 and the minute backward-pointing hooks studding the 

 angles of the stems and the margins of the leaves in 



