MICROSCOPIC OBJECTS. 



Fig. 17. Spiral-fibre Cells from a Cactus, x 120. 



The bundles and vessels of spiral-vascular tissue, in some 

 cases, exhibit the fibre as consisting of two or more spirals, 

 although it is usually single, as in the drawing of the vessels 

 of the Rhubarb (fig. 16). But in the drawing we are now 

 considering, the pointed fibre-cells must be especially noticed. 

 One of them will be seen with the spiral fibre imperfectly 

 formed; from some cause the secondary deposit has not 

 developed a complete spiral, but formed a dotted cell instead. 

 An annular vessel will also be seen on the left-hand side at 

 the bottom of the drawing. 



This object may be mounted dry or in liquid. 



Fig. 18. Section of Vegetable-Ivory Nut (Phytelephas 

 macrocarpa), x 120. 



This section of the Vegetable-Ivory nut is drawn to illus- 

 trate the thickening of the secondary deposit alluded to at 

 page 3 (fig. 5). When a section of this nut is made, and 

 ground down until it is very thin, and then mounted dry or 

 in balsam, it will exhibit bodies something of the shape of a 

 compressed cheese-mite (see Mites) . This peculiar appearance 

 is caused by the irregular thickening of the cellulose of the 

 cell-wall, called secondary deposit (see also fig. 48). 



Cells that are to exist for a long time in any structure 

 always have their cell-wall thickened until it becomes of a 

 bony or horny nature. That this deposit has been gradually 

 formed, may be demonstrated by maceration, or the applica- 

 tion of sulphuric acid, which resolves the thickened cell- wall 

 into different layers. 



This object is best mounted dry ; but it may be mounted 

 in fluid. 



