50 



STEMS 



made by plants. A hop vine which has climbed to the top of 

 its stake may sweep its tip continually around the circumfer- 

 ence of a circle two feet in diameter, and the common wax 

 plant (Hoya) of the greenhouses sometimes describes a five-foot 

 circle, the tip moving at the rate of thirty-two inches per hour. 1 



This circular motion is 

 produced by unequal 

 growth of the two sides 

 of the stem. 2 



The direction in 

 which twiners coil 

 about a supporting ob- 

 ject is almost always 

 the same for each spe- 

 cies of plant, but not 

 the same for all species. 

 In the hop it is as shown 

 in Fig. 42, but in many 

 plants the movement is 

 in the reverse direction. 

 65. Short-stemmed 

 plants. As will be 



FIG. 43. The dandelion, a short-stemmed plant shown later (Chapter 



xxxiv), plants live sub- 

 ject to a very fierce competition among themselves, and they are 

 exposed to almost constant attacks from animals. 



While plants with long stems find it to their advantage to 

 reach up as far as possible into the sunlight, the dandelion, the 

 cinquefoil, the white clover, some spurges, the knotgrass, and 

 hundreds of other species, living in open places, have found 

 safety in hugging the ground. The dandelion, fall dandelion, 



1 See article on " Climbing Plants," by Dr. W. J. Beal, in the American 

 Naturalist, Vol. IV, pp. 405-415. 



2 See Strasburger, Noll, Schenck, and Karsten, Text-Book of Botany, 

 pp. 281-284, London, 1908. 



