166 A TEXTBOOK OF GENERAL BOTANY 



Friq.163. Branches of Bougainvillea modified 
as spines used in climbing, (x #) 
light and bear large num- 
bers of leaves without 
the necessity of expend- 
ing the material which 
would be necessary to 
build a stem sufficiently 
strong to support the 
leaves and reproductive 
organs by its own strength. 
A climbing plant depends 
on the strength of some 
other object for its sup- 
port, and so can develop 
long, slender stems. It is 
probably because of their 
slender stems that climb- 
ers usually grow rapidly and so can shoot up through dense 
vegetation and reach the light more quickly than can a plant 
which has to develop a stem thick enough to support itself. 

Fic. 164. Stems of Euphorbia tirucalli specialized for photosynthesis and 
water storage. (X 3) 
