40 



POPULAR HISTORY OF LICHENS. 



polygonal, oblong, or irregular in different species ; and the 

 cylindrical or elongated cell, by the disappearance of whose 

 septa, when in apposition to other cells, the tube or filament 

 is produced. These medullary tubes, though bearing a ge- 

 neral resemblance, have no functional affinity to the vascular 

 tissue of the Phanerogamia : they are not vessels or canals 

 for the conveyance of sap, though they imbibe and convey 

 water with great readiness and rapidity. The walls of both 

 forms of cell above-mentioned are composed of cellulose, a 

 non-nitrogenous substance resembling starch; and Payen 

 remarks that the cell-wall in Lichens differs from the thick 

 sclerogenous cell-wall of the seed of the Phyteleplias, or 

 Ivory Palm, merely in its superior thinness. Between the 

 individual cells in the various tissues there is an intercellular 

 matter of a gummy nature, which is supposed by some au- 

 thors to be an excretion of, or product thrown out by, these 

 cells. 



Of all the cell-forms above described, the most important 

 are the isolated cellules of the sub-cortical or gonidic layer, 

 which are denominated Gonidia (yovTj, generation, and elSos, 

 resemblance, probably so called from their functional re- 

 semblance to the spores"*). Prom the important part they 

 * Vide Korber 'De Gonidiis Lichenum and also in the 'Annales des 



