239 
but are closely imbedded in the plant-tissue, either singly or 
two or three together ; in the last case often united side by 
side, as if by partial fusion of their shafts, and seldom easily 
separable from each other. They are commonly much larger, 
and occasionally smaller, than raphides ; but, however small 
the crystal prisms may be, they are easily distinguishable by 
the characters above specified from raphides, and yet these 
two different kinds of crystals, as well as sphaeraphides, are 
still commonly confounded together under this lastname, and 
perpetual mistakes and perplexities occur accordingly. 
Among our Exogens good examples of crystal prisms must 
be rare. They are small, but distinct when highly mag- 
nified, in the ovary-coat of many Composite, as the Thistles, 
Saw-wort, and Knapweed, and somewhat larger in the 
Milk-Thistle. These little prisms must not be confounded 
with the still smaller, but square, cuboid, tubular, lozenge- 
like, and other forms, common in these plants, and, indeed, 
throughout some of the tissues of the whole Phanerogamia. 
Certain closely allied plants may be easily and surely distin- 
guished by one or other of them and the crystal prisms. 
Thus, while the shape of the prisms in the ovary-coat of 
Centaurea nigra is very distinct, and their length To Voth of 
•an inch, with a thickness of ■ 5 - 4 ‘ o6 th, in the same part of C. 
ragusina and C. Scabiosa they are not thus elongated, but 
appear more or less square, cubical, or lozenge-shaped, and 
about g-jig-th of an inch in diameter. But magnificent 
crystal prisms are afforded by many exotic Exogens, of which 
excellent examples occur in the common officinal barks of 
Quillaja and Guaiacum. 
Such, too, are frequent in Endogens, as the old garden 
favourite Iris Germanica, and many other Iridacese, including 
the Orris Root of the shops. In Fourcroya, an Amaryllid, 
the crystal prisms are still more remarkable. They some- 
times appear with raphides in one and the same plant, as 
may be witnessed in other Amaryllids, as well as in certain 
Orchids and more orders. All the large crystal prisms are 
admirably adapted for experiments with polarized light. The 
bulb-scales of the Onion, Shallot, Garlick, and Leek, abound 
in smaller prisms, many crossing each other, and also affording 
pretty objects for the polariscope ; but I have never seen true 
raphides in the Allicae. 
IV. Conflicting Statements. — Remarkable is the po- 
tency of error. Here it has arisen and prevailed partly from 
insufficient observations, but chiefly, as before noticed, from 
the general practice of confounding several distinct kinds of 
