IRRITABILITY 245 



alternately the steep spirals typical of twining plants and 

 short close spirals like tendrils. After it has sent haus- 

 toria into the tissues of its host, dodder elongates very 

 rapidly for a few hours. During this time the stem is not 

 sensitive to contact and it behaves like an ordinary twining 

 plant, circumnutating and obeying the force of gravity. 

 Presently, however, the rate of growth decreases and then 

 it begins to be sensitive to contact. When irritated by con- 

 tact with an object of suitable size, dodder will make two 

 or three or even more close tendril-like turns . about the 

 support. It will not form these close coils about moist 

 gelatine, it cannot twine about objects too large or too 

 small. The longer the contact and the rougher the surface 

 of the support, the more prompt and pronounced will be the 

 bending. Only prolonged contact will induce permanent 

 bending. This is the first effect induced by contact. 



A second effect consists in the formation of haustoria, 

 lateral root-like organs which the parasite sends into the 

 tissues of its host and through which it draws needed food. 

 Without contact these organs never form, even in rudimen- 

 tary conditions. Without contact with an object able to 

 furnish food as well as mechanical support to the parasite, 

 the haustoria will not fully develop. Contact stimulus in- 

 duces the stem to bend closely about the support and to 

 form haustoria, but chemical stimulus is needed besides 

 to secure the development of the haustoria. In fact, the 

 seedling dodder will not even twine about innutritions 

 supports. Furthermore, the dodder stem is sensitive to 

 gravity and will not twine closely or otherwise about a 

 suitable, even nutritious, support unless the position of 

 this be vertical or only slightly inclined. 



The dodders, and a few tropical plants like them ( e. g. 

 Cassytha), are exceptional twining plants. The great 

 majority of twining plants are not sensitive to contact, 

 and though they twine about vertical or nearly vertical 

 supports of suitable size and form, they do so by the com- 

 bined action of their spontaneous nutation movements 

 and of gravitation. As Darwin so clearly demonstrated,* 

 * Darwin, C. The power of movement in plants. 



