COMPARED WITH THOSE OF ANIMALS. 65 



or opaque points on the cell wall. The scalariform and 

 porous varieties of ducts are thus generated. That all 

 these differences in the fibro-vascular tissue are only modi- 

 fied forms of the spiral deposit, is proved by the occasional 

 presence of the spiral, the annular, the scalariform, and 

 even the dotted varieties of deposit in the same vessel. 



The porous or dotted cells which form the tubular tissues 

 termed by botanists bothrenchyma, are not, however, re- 

 ferable to the fibrous type, from which they differ totally 

 in their original formation. These cells are endowed with 

 the peculiar property of restricting the earthy deposit to 

 certain portions of their parietes ; the thickening process 

 continues at these points, the other parts of the cell-wall 

 remaining uncovered. The dotted appearance of the cell- 

 wall results therefore in this case, not from the presence 

 of an opaque deposit, but from its absence. The pits, or 

 cavities of contiguous cells, frequently correspond with each 

 other notwithstanding the thickening of their walls, as is 

 clearly shown in the wood of the American plane tree. 

 These facts prove that the earthy matter is controlled in its 

 deposition on the cell-wall, by other influences than those 

 which are purely mechanical, and that the phenomena is 

 to be attributed to a peculiar vital action, originating in 

 the individuality of the cells. 



To the same cause is to be attributed, the different 

 varieties of canaliculi or radiating tubes in the more indu- 

 rated deposits, termed sclerogeu. The stony envelope 

 which surrounds the seeds of the peach and plum, together 

 with the shell covering of nuts and filberts may, in this re- 

 spect, be fairly contrasted with the more solid and indurated 

 tissues of animals. Bone, for example, exhibits the same 

 lacunae, canaliculi, and radiating processes, amongst other 

 peculiarities, "In the Histological Catalogue of the 

 6* 



