THE MOVEMENTS OF PLANTS. 



37 



is a continual " bowing around " of the apex.* At the same time 

 gravity is always trying to pull the tip to the ground. This in 

 turn tries to stiffen itself by putting on tissues of wood in opposi- 

 tion to the strain. Then, if the plant happen to be unequally 

 illuminated, there will be another " pull " in the direction of the 

 stronger source of light. The result is that the shoot moves in 

 irregular ovals, zigzags, &c. 



As a leaf-climber, Solatium jasminoides may be taken. 

 Darwin tells us that a long shoot made four revolutions against 

 the sun at an average rate of 3 hrs. 26 mins. In a hothouse a 

 stick was clasped by a petiole in seven hours. After three or 

 four days the petiole increased much in thickness, the three 

 isolated fibro-vascular cords becoming united and forming a 

 cylinder like that in a stem. This increase is due to response to 

 the strain felt. An analogous but not identical growth is seen 

 in a horse-chestnut petiole ; for in this there is a complete cylinder 

 of wood, but the three cords, usually only occurring in petioles, 

 are embedded in the middle of the pith. In all cases the extra 

 growth of wood indicates the amount of strain to be met by the 

 petiole. 



Gloeiosa. — This plant has a stem which describes an 

 irregular oval in about three hours. The leaves are horizontal, 

 the end of each forming a slender hook-like extension. If it 

 catch anything, it at once anchors the plant, which is somewhat 

 over-weighted above, and stops the circumnutation. The inner 

 surface of the hook is sensitive, so that in from one to three hours 

 it curls inwards and permanently seizes an object in from eight 

 to ten hours. If it does not catch anything, after a time the tip 

 curls up into a helix and loses its sensibility. 



Tendkils are the results of modifications of more than one 

 kind of organ. They are generally metamorphosed leaflets ; 

 but in vines and Ampelopsis they are homologous with flowering 

 branches. Strychnos may have its branches in the form of 

 tendrils ; while Dissochceta has adventitious roots similarly 

 metamorphosed. Circumnutation coupled with a sensitiveness 

 to contact are the usual means by which these organs effect 

 their purpose. 



Veenation. — This term is given to the way the young leaves 



* If a hazel shoot be split vertically, the surfaces are never flat, but 

 show a twist. 



