VARIATIONS IN TEMPERATURE 251 



Similar response to contact with a neighbouring foreign body 

 is met with in the sensitive petioles of certain climbing species 

 of TropcEolum and Solanum, and in a lesser degree is observable 

 in many twining and climbing stems. 



Small portions near the tips of roots are sensitive to prolonged 

 lateral contact : when such parts touch stones and other hard 

 objects in boring their way through the soil, they curve away 

 from the irritating bodies and the root tips continue their 

 growth in a new direction. 



On the other hand the older portions of growing roots when 

 stimulated by contact curve towards and endeavour to grow 

 round the irritating objects. 



Both these and the nutating movements previously mentioned 

 are such as enable roots to pass obstructing objects in their path. 



Ex. 151. — (i) Observe the form of the free tendrils of the vetch, pea, vine 

 and white bryony {Bryonia dioica L.). Compare with tendrils attached to 

 their supports. 



(ii) Arrange so that some of the free tendrils which are about three parts 

 grown shall come in contact near their tips with small twigs or other similar 

 support. Examine at intervals of a few hours and note the amount of 

 twining of the tendril round its support. 



(iii) Stimulate the concave side of the curved end of a tendril of white 

 bryony, encumber or melon for about a minute by rubbing with a moderately 

 smooth piece of wood, and then watch its subsequent behaviour for two or 

 three minutes. Does its curvature increase ? 



Ex. 152. — Examine the mode of climbing of Solanum iasminioides. 



(ii) Movements in response to variations in temperature, and 

 the changes of day and night. — Tulips, crocuses and other 

 flowers open on a warm day or when brought into a warm room 

 and close when placed in a cool situation. The opening and 

 closing movements go on independently of light and are brought 

 about by alterations in the growth and turgidity of the cells 

 forming the upper and lower sides of the petals ; the change of 

 temperature stimulates the protoplasm in such a manner that 

 varying amounts of water are allowed to pass through it into 



