144 PART II. — VEGETABLE HISTOLOGY. 



the rhizome of Trillium grandiflorum. The curious stalked bodies 

 called cystoliths, that occur in large cells just beneath the upper 

 epidermis in the leaf of Ficus elastica, Fig. 389, and in some other 

 plants, consist mainly of calcium carbonate. 



Besides the classes of substances already described as occur- 

 ring in the cell-sap, or deposited in the cell-walls of plants, there 

 are various others of less importance, such as certain coloring 

 matters, aromatic compounds and decomposition products, 

 which are as yet too imperfectly understood to be properly 

 classified. 



Practical Exercises. 



1. Strip off the epidermis from the upper side of one of the fleshy scales 

 of the onion bulb, and examine it with a magnifying power of about 200 diam- 

 eters. It will be seen to consist of a layer of elongated cells. The cell-walls, 

 though transparent, are distinctly visible, but the protoplasm is difficult to 

 detect. With care, however, the nucleus and some minutely granular matter 

 may be seen in the interior of the cell. Now treat the tissue with a drop of 

 nitric acid, and then with a little ammonia. The protoplasm will be rendered 

 distinctly visible as a granular mass in the interior of the cell. The re-agents 

 have also caused it to shrink away from the cell-wall, which now appears 

 thinner than before, because the primordial utricle is no longer in contact with it. 



Take now a fresh portion of the same tissue, treat it for a few moments 

 with a strong solution ot iodine, rinse it for a moment in clean water, and 

 examine with the same magnifying power as before. The cell wall will be but 

 slightly stained, the protoplasm will have acquired a distinct brownish-yellow 

 color, and the primordial utricle, with its two layers, and the nucleus, nucleoli 

 and vacuoles, will be distinctly recognized. 



Now bring a drop of strong sulphuric acid into contact with the iodine- 

 stained tissue, and immediately observe the result. The cell-wall will be 

 stained blue. The sulphuric acid first converts it into amyloid, a substance 

 closely allied to starch, and the iodine then reacts upon it to produce the blue 

 color. This constitutes one of the best tests for cellulose. The cell wall will 

 in a short time be completely dissolved by the acid, but the protoplasm, though 

 finally destroyed, disappears much more slowly. Lastly, draw a few of the 

 cells, so as to show the structure and relation of parts. 



2. Take a small piece of the green leaf of the Fresh Water Eel-grass, Val- 

 isneria spiralis, and examine the cells with a magnifying power of 300 or 400 

 diameters. Among the cell contents will be seen numerous green granules, the 

 chlorophyll-bodies. They are in motion, being conveyed around the interior of 

 the cell-walls by means of the currents in the protoplasm. Study these move- 

 ments with care, and then treat the cells with iodine solution. After a few 

 minutes, wash off the superfluous iodine and then examine. The protoplasm 

 has not only been stained, but its movements have been stopped and its life 



