[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 259 



Fig. 93. Thorns on honey locust. 



which the plants are growing. In this way the vines can "cUmb" vertical 

 walls. In the morning glory, the hop, and some beans the stem itself 

 winds about the support and may be called a twiner. It is interesting to 

 note that some plants twine clockwise and others counterclockwise, when 

 viewed from above. No explanation of this difference in twining has been 

 established. 



Cladodia. This type of stem is found among the cacti and some other 

 desert plants. Leaves are usually absent or ephemeral, and spines often 

 develop on the green stems. Such stems are referred to as cladodia 

 ( singular cladodiiim ) . 



Dormancy in buds. It is a well-known fact that mature buds of most 

 woody plants, those in the "eyes" of potatoes and in bulbs pass through 

 a dormant period after maturity. During this time there is no external 

 evidence of growth, regardless of the environment in which the buds 

 are placed. Such dormancy is the result of physiological conditions 

 within the buds, and does not disappear until these inner conditions 

 have changed. In temperate climates it may persist for several months. 

 It is often commercially important for this dormant period to be short- 

 ened as much as possible. 



Most dormant buds will grow much sooner than usual if they are sub- 

 jected to various artificial treatments that accelerate certain physiologi- 

 cal changes in them. These treatments are referred to as methods of 

 "breaking dormancy." In temperate climates the internal conditions that 

 cause dormancy in buds disappear under the influence of the low 



