Page:An introduction to physiological and systematical botany (1st edition).djvu/69

Rh. 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 various parts, but not performing any remarkable office in the vegetable œconomy.

Linnæus, 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 not 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.