ORGANS OF NT7TBITI0N. 



89 



of separate portions. Such stems when examined at their first 

 development present the usual endogenous structure, but in 



Fig. 177. The Douni Palm of Egypt {Hyphwiie thehaica). 



consequence of tlieir gro-vv-th in diameter taking place more 

 rapidly than new matter can be deposited in their interior, they 

 soon become hollow. 



In the stems of some other monocotyledonous plants we have 

 a more striking dcAiation from the ordinary structure. Thus 

 the Sarsaparillas and a few other allied plants have aerial stems 

 which are strictly endogenous in structure, and underground stems 

 which have the vascular bundles arranged in one or two zones 

 around a central parenchyma {Jiff. 180), like the wood about the 

 pith of an exogenous stem: such vascular bundles have, however, 

 no cambium -layer like those which form the zones of an exoge- 

 nous stem, and have consequently no power of indefinite increase 

 like them. 



There is nothing in the internal structure of endogenous stems 

 by which we can ascertain their age as we can those of exogenous 

 structure. It is supposed that the age of a Palm stem is indi- 

 cated by the annular scars which are produced on its external 

 surface by the fall of the terminal tuft of leaves, for as one tuft 

 only is commonly produced annually, each ring marks a year's 

 growth, and hence the number of scars corresponds to the num- 

 ber of years the plant has lived. Although it is true that in some 

 few cases such a rule may enable us to ascertain the age of a 



