HUTCHINSON'S POPULAE BOTANY 



FIG. 53. SEC- 



layer of cork cells. The thick, rough, 

 cleft bark of a Spanish species of Oak 

 (Quercus suber) is the cork of com- 

 merce, of which the stopples for bottles 

 and casks are made. It is stripped off 

 without injury to the stem which, 

 > indeed, soon gets covered with fresh 

 layers of corky cells, and in eight 

 or ten years the tree is again ready TION of a Part 

 FIG. 52. CELL from the for stripping. The first peeling of a Pitted Cell 



Ba C k onifer a ^docarp^ takeS P laCG whe11 the tree is twent J- ( dia ^ ammatic )' 



dacryoides). five or thirty years old, and great 



care is always taken not to injure the inner bark. 

 Earthy or mineral substances, found in all plants, abound in some forms 

 of secondary deposit, and may be readily detected when any part of the 

 plant containing them is burned. The ash left after burning is commonly 

 known as " the ash of plants," and consists chiefly of silica, lime, and 

 magnesia. Silica (flint) is particularly plentiful in the grasses, canes, etc., 

 the glassy appearance of the stems of such plants being due to the presence 

 of this mineral. Years ago, a melted mass of glassy substance at first 

 supposed to be a meteoric stone was discovered in a meadow between 

 Mannheim and Heidelberg in Germany ; but when chemically examined it 

 was found to consist of silex combined with potash. Upon inquiry it was 

 ascertained that a stack of hay, which had been recently destroyed by 

 lightning, had stood on the spot. The siliceous mass was simply the ash 

 that remained after the conflagration. One cannot reduce haystack burning 

 to a system for purposes of experiment, 

 but instructive results may be obtained 



on a small scale by igniting 



a piece of siliceous tissue on 



platinum foil, after soaking 



in nitric acid. If the ash is 



then treated with the same 



acid, it will show an insoluble 



residue, and that residue is 



flint. 



It frequently happens 



that the protoplasm deposits 



secondary thickening only in 



some parts of the cell-wall, 



FIG. 55. DIAGEAM to illustrate the 



FIG. 54. 

 PITTED 

 Wood Cells 

 from a Big- 

 nonia. 



the other portions being left 

 bare. For this reason we 



post in Porous Cells, (p) Pores. The 

 Broken Rings represent Successive 



Layers of Secondary Deposit. The Pro- 

 toplasr 



Thus, in what are Space. 



get some curious varieties of toplasm occupies p art ~ of the Central 

 cells. 



