IRRITABILITY 241 



in such cases among plants are due to contact, though it 

 seems to be the case in animals.* 



Among higher plants, familiar examples of growth stimu- 

 lated by contact are afforded by those species of Ampelopsis 

 which form the curiously branched organs of attachment 

 shown in the accompanying figures {A. Veitchii). The 

 young branches, "ten- 

 drils," (a) are soft, 

 weak, tapering, long 

 and slender, slightly 

 enlarged at the tips 

 and green only there. 

 After the tips come into 

 contact with a suita^ 

 ble vertical surface, they 

 broaden and flatten out, ^'S^^ 19.-Tendrils of Ampelopsis Veitcbii. 



, , . , a, before, b, after attachment, 



pressmg closely against ' 



the surface, forming the little discs which presently be- 

 come very strongly attached (&). If the "tendrils" do 

 not touch a surface suitable for attachment, the tips 

 do not enlarge, the whole organ remains weak, and 

 finally dies and falls away. Stability of the support and 

 prolonged contact with it seem to be indispensable to the 

 formation of the disc-shaped bodies. These are the larger 

 the rougher the surface of the support, other things being 

 equal. The subsequent thickening and strengthening of the 

 "tendril" are due to mechanical pull (see pp. 187-8) rather 

 than to the contact of the tips with a rough object. 



The direction of growth is controlled in many instances 

 by contact with solid objects. Thigmotropism or stereo- 

 tropism is the name proposed for this phenomenon. The 

 most striking examples are furnished by tendrils,t which 

 have been studied by many observers. 



The sensitiveness of tendrils to contact varies greatly 

 with the species, the prevailing conditions, and the age 



* Loeb, J. Untersuchungen zur phyaiologischen Morphologie der Thiere. 

 1. tjber Heteromorphose. Wiirzburg, 1891. 



f See Darwin's "Movements 'and Habits of Climbing Plants" and Pfef- 

 fer's P"anzenphysiologie Bd. II.. 2ter Theil 2te Auflage. 

 16 



