HALF-AN-HOUR AT THE MICROSCOPE. 49 



ammonia is neutralised, or until the solution changes from a lilac 

 to a scarlet colour.— N.B. Keep a good supply of injections 

 ready in vessels convenient to heat, having wide mouths for the 

 syringe to enter. Occasionally filter to remove any particles of 

 matter which may get into them, see that they are labelled 

 distinctly, and ever clean, and in good order. 



1balf^an*1bour at tbe flDicroscope 



Mitb /IDr. Zxxttcn Mest, jf.X.S., jf.lR./ID.S., etc. 



Longitudinal Section of Alder (PI. III., Figs, i— 6).— The 

 section from which the drawing was made was taken from near the 

 surface, and hence shows the numerous viedtiUary rays, which are 

 lines of cells, wedge-shaped in transverse section, proceeding 

 horizontally to the surface ; their purpose is to keep up vital con- 

 nection between the inner and outer structure. On examining a 

 section we see well the division of the bark into its three com- 

 ponent parts— outer, middle, and inner. The outer bark, or 

 corky layer, is protected, in young branches, by a single thickness 

 of cells, which occasionally possess stomata — the " epidermis " ; 

 the middle layer is composed of cells containing chlorophyll, to 

 which the green colours of this part is due. The inner layer is 

 called the " cambium region " ; it is here that in its ascent and 

 descent, new structure is continually being formed. Between the 

 middle and outer bark is the " fibrous layer " ; in the alder 

 crystal-prisms occur here, and cells having a very close 

 resemblance in their structure to bone, and scarcely to be 

 distinguished therefrom, save by the presence of the surrounding 

 cell-wall. "Bone-cells" of this kind are not common in wood; 

 indeed, I do not, whilst writing, recall another instance of their 

 recurrence in that part ; they form the structure of the stones of 

 fruits, such as the cherry, which presents them in great perfection, 

 and the shells of nuts ; the outer husk of hemp-seed, and the 

 gritty substance of pears, furnish good examples of the structure. 



An interesting condition of the pitted ducts is present in the 

 alder — that is, continuous bars across, forming an approach to 

 scalariform tissue, which is so strikingly shown in an oblique 

 section of Pteris aqtiilma. 



On examining fresh alder-wood, the interesting fact was found 

 that sphffiraphides occasionally replaced the prisms in the bark of 



Vol. VI. E 



