ELEMENTARY STEUCTUEE. 



23 



These discs occvir either in single rows {^ig. 38), or in double 

 {figs. 42 and 43), or in triple rows {fig. 44). In those cases 

 where there is more than one row of discs, the discs in each row 

 may be either on the same level, as is more commonly the case 

 {fig, 42), or at different levels, and hence alternate to each other, 

 as in the Araucarias and allied trees {figs. 43 and 44). 



Fig. 41. 



1/1 



Fig. 42. 



Fig. 43. 



Fig, 44. 



Fig. 41. Wood-cells from niicitcm ani^atum. From Gray. 



Fig. 42. Disc-bearing wood-cells of the Pine, witU a double 

 row of discs, which are ou the same level, or opposite to 



each other. After Nicol. Fig. 43. Disc-bearing wood-cells 



of AUingia exc^lsa, with double rows of discs, which are 

 alternate with each other. Fig. 44. Disc-bearine wood- 

 cells of Araucaria, with double and triple rows of alternate 

 discs. After Nicol. 



The cause which leads to the formation of these lenticular 

 cavities has not yet been clearly proved. According to Schleiden, 

 they arise from the occurrence of bubbles of air between the walls 

 of the cells ; but this must be incorrect, for such cavities, as first 

 shown by Mohl, are filled with sap in the young condition of the 

 cells. 



Fibrous Cells. — It frequently happens that the secondar}- layers 

 (instead of being deposited in the form of perforated membranes, 

 which give rise to the pitted cells just described), consist of delicate 

 threads or bands of varying thickness called fibres, which assume 

 a more or less spiral direction upon the inner surface of the pri- 

 mary cell-membrane {figs. 45-47), and thus give rise to what are 

 called fibrous cells. Such fibrous cells occur in various plants and 

 parts of plants; thus in the leaf of the Sphagnum, the hairs of 

 many Cacti, in some of the membranes of many seeds, as those 

 of Salvia and Collomia; in the spore-cases of some of the Flower- 

 less Plants, in the inner lining of all anthers, in the outer rind of 

 the aerial roots of many Orchids, and in several other instances. 



These fibrous cells also present some differences of appearance 

 as regards the distribution of their fibres. Thus, in some cells 



