30 



ELEMENTARY ORGANS OP PLANTS. 



the sclerogen has not been deposited, and, consequently, visi- 

 ble by the greater transparency of the tubular walls in that 

 part. The disks, or saucer-like depressions on the walls of two 

 contiguous tubes, are applied to each other, whereby a small 

 space is left between them, similar to the cavity left between 

 the concave surfaces of two watch-glasses. In some fossil 

 woods, pieces of silica, like double convex lenses, have been 

 removed from these cavities, so as to leave no doubt as to the 

 nature of these interspaces between the fibres, of which they 

 were clearly the casts. 

 29. These circular markings in the fibres of pine woods are, 



Fig. 7. 



■ I. e. 



therefore, caused by cavities formed by the concave depres- 

 sions or lenticular cavities, I, c, between the walls of two con- 

 tiguous tubes, the centre of each depression being thin and 

 transparent, and producing the dot seen in the centre of the 



