2o6 TEXTBOOK OF BOTANY 



causes the tendril to grow more slowly at that point. 

 Gro^¥th continues as before on the opposite side of the 

 tendril, and as a result the tendril curves toward the object 

 that it touches. As new points on the tendiil come into 

 contact with the object about which it is twining, growth 

 is checked in the same way at those points, and so the 

 curling continues until the tendril ceases to grow. Then 

 the part of the tendril below the point at which it began to 

 curl about the supporting object coils like a spring. This 

 shortens the tendril and pulls the part of the plant from which 

 it grows tightly against the support. 



In the Virginia creeper and in some other plants (Fig. 128), 

 when the end of a tendril touches a sohd body it broadens 



out into a disk that fits closely 

 into the cracks and uneven places 

 in the support and so helps to hold 

 the plant in place. These disks 

 enable the plant to climb up the 

 face of a wall or of a rock where 



Fig. 120. — .^ branched thorn , . , . , ., , 



of the honey locust. there IS no object slender enough 



for the tendrils to coil about. 

 Thorns, like tendrils, are sometimes branches and some- 

 times leaves. They protect the plant from injury by 

 making it unpleasant for animals to brush against or to 

 eat it. Branch thorns are either simple like those of the 

 hawthorn, the pear, and the blackthorn, or branched like 

 those of the honey locust (Fig. 129). Many chmbing plants, 

 especially in the tropics, are assisted in climbing by thorns 

 which are turned or curved backward and which catch in the 

 trunks of trees. 



225. The Strength of Stems. — Every erect stem that 

 grows above ground is subject to strains that are severe in 

 relation to the size of the plant. These strains result partly 

 from the weight of the stem itself and of its branches, leaves, 

 and flowers, which in the case of large trees amounts to 



