104 FUNCTIONS OF ENDOGENOUS STEMS. 



contained in the stem, that they are no longer pervious to fluid, 

 and the tree dies. Nature sometimes provides a remedy for this, 

 in the splitting of the hard envelope, which allows the interior to 

 dilate ; and this has been successfully imitated by art, vigour 

 having been restored to a Palm which had begun to languish, by 

 splitting down the exterior of its stem with a hatchet. 



143. Although no complete distinction exists between the 

 cellular and woody portions of the stem in Endogens, yet it is in 

 the interior that the former predominates ; and hence some Palm- 

 trees, as that which produces Sago, have been said to have a pith, 

 which is (strictly speaking) erroneous. Through what channels 

 the ascending and descending sap of Endogens respectively move, 

 has not been ascertained ; but there can be little question, that it 

 is chiefly through the ducts that the former rises, whilst the 

 latter finds its way downwards through the cellular interspaces. 

 From the cellular portion of the stem, in Endogens as in Exogens, 

 the buds take their origin ; whilst the roots are chiefly prolonga- 

 tions of the bundles of wood and vessels. This is well seen in 

 the Pandanus, where the bundles that ordinarily make their way 

 downwards within the stem, and would there form part of the 

 wood, pass outwards and become aerial roots (. 110). 



144. The same cause which sets a limit 

 to the age of Endogens, exempts them from 

 the injurious effects, which Exogens often 

 experience, from the compression of their 

 stems by ligatures of various kinds. If 

 a cord be tied tightly round the trunk of 

 an Exogen, it will offer little impediment 

 to the ascent of the sap ; but it will ob- 

 struct its descent through the bark. In 

 consequence, there will be a deficiency of 

 nourishment to the parts beneath, and a 

 superfluity above; so that a protuberance 

 will arise from the stem, just at the point 

 where the downward flow of the sap is 



checked. This protuberance will increase in progress of years, 

 if the tree survive, so as almost to bury the cord beneath it ; but 



