u 



ORGAKOGRAPHT. 



These discs occur either in sinijlerows (Jig. 21), or in double 

 (fig. 25), or in triple rows (fig. 28) ; in the latter cases the discs 

 in each row may be either on the same level, as is more com- 



Flg. 24, Fig. 25. Fig, 26. 



Fig. 28. Fig. 27. 



Fig. 24. Wood-cells from nudum floridannm. From Gray. 



Fig. 25. Disc-bearing wood-cells of the pine, with a double row of discs. 



After Nicol. Fig. 2<). Disc-bearing wood-cells of Ahingia excelsn, with 



double rows of discs, which are alternate with each otlier Fio. 27. Disc- 

 bearing wood-cells of the Weymouth pine il'inus Stmhiis), with double 

 rows of discs, which are on the same level, or opposite to eacli other. 



After Nicol. /•'/'/. 2S. Disc-bearing wood-cells of Araucaria, with 



double and triple rows of alternate discs. After Nicol. 



monly the case (figs. 2.5 and 27>, or at different levels, and hence 

 alternate to.cach other, as in the Araiicarias and alhed trees (fiffs. 

 26 and 28). These discs are commonly found in the sides of the 

 wood-cells which arc tunicd towards the silver-grain of trees 

 (see p. 83); while the ordinary pits of wood-cells are generally 

 more abundant on the sides looking towards the centre and 

 circumference of the tree. 



The cause which leads to the formation of these lenticular 

 cavities has not yet been clearly proved. According to Schlei- 

 den, they arise from the occurrence of bulibles of air between the 

 walls of the cells in which they are fouiul ; but Mohl states 

 that such an assertion is incorrect, for such cavities are filled 

 with sap in the young condition of the cells, which indeed may 

 be seen by any one with ordinary care. 



Fibrous Cells. — It frcf|uently bapi)cns that the secondary 

 layers (instead of being deposited in the form of perforated 



