OF THE MEDULLA OR PITH. 39 



ties. Many grasses and umbelliferous plants, 

 as Conium maculatum or Hemlock, have 

 always hollow stems, lined only with a thin 

 smooth coating of pith, exquisitely delicate 

 and brilliant in its appearance. 



Concerning the nature and functions of 

 this part various opinions have been held. 



Du Hamel considered it as merely cellular 

 substance, connected with what is diffused 

 through the whole plant, combining its vari- 

 ous parts, but not performing any remarkable 

 office in the vegetable oeconomy. 



Linnaeus, on the contrary, thought it the 

 seat of life and source of vegetation ; that its 

 vigour was the main cause of the propulsion 

 of the branches, and that the seeds were 

 more especially formed from it. This latter 

 hypothesis is net better founded than his idea, 

 already mentioned, of the pith adding new 

 layers internally to the wood. In fact the 

 pith is soon obliterated in the trunks of many 

 trees, which nevertheless keep increasing, 

 for a long series of years, by layers of wood 

 added every year from the bark, even after 

 the heart of the tree is become hollow from 

 decay. 



