CELLULAR TISSUE. 15 



of the neighbouring Clostres. They are filled with a 

 particular matter, more watery in young wood than in 

 old ; and the nature of which determines the hardness, 

 the weight, the varying colour of different woods com- 

 pared together, and of the same wood at different periods 

 and from different parts of the same plant. The tissue, 

 described by Link, under the name of the Tissue 

 of the Alburnum (tissu d'auhier), comes under this 

 class. 



2d. There are other cellules to w^hich the name of 

 Little Tubes (tuhilles), (PI. 1, fig. 8; PL 2, fig. 1, 3) 

 proposed by Cassini, will be tolerably well adapted : 

 these are cylindrical, or iwismatic, and are not swelled 

 out in the middle of their length. They are always 

 found around the vessels of vascular plants ; and they 

 alone compose the nerves, peduncles, and stems of plants 

 destitute of vessels, as the Mosses and Algas. It must 

 be remarked, that in several of these cellular plants, as 

 the Mosses and Hepaticas, there is suddenly a remark- 

 able change of figure among the elongated cellules which 

 form their nerves, and the round cellules which compose 

 their parenchyma ; whilst in vascular plants there often 

 is an insensible transition of form, of elongated cellules 

 which surround the vessels, to the round cellules Avhich 

 form the parenchyma. Rudolphi concludes from this, 

 that the elongated cellules of Mosses might well be 

 considered as a particular class of vessels; but this 

 opinion does not appear to us to be sufiiciently proved 

 by this sole consideration. 



Lastly. There is a final order of cellules, which, in- 

 stead of being lengthened in a longitudinal direction, 

 are elongated transversely (PI. 1, fig. 9, 10) : these are 

 the cellules which compose the medullary rays, and 

 which are necessarily peculiar to the class of Dico- 

 tyledons. Kieser, who first proposed to distinguish 



