262 Crystals in Testa and Pericarp of several Orders of Plants, 



of water or glycerine on the object-plate. Thus the crystals will be 

 quickly and easily found ; and in the fibrous bundles of the sutures 

 of the green pea-pod, they may often be detached by comminution 

 of the part from their seat, so as to be made to roll over and display 

 their forms in the microscopic field of vision. Not so in the tough 

 inner skin of the pod-valve of this plant, among the fibres of which 

 the crystals are thickly studded, and so firmly fixed in and hidden 

 by this dense tissue, as not to be easily seen therein or detached 

 therefrom. But by drying it, and then scraping it with a knife in 

 a drop of turpentine, the texture is made more transparent, and 

 some of the crystals may be found floating freely and separately 

 around : and indeed plant-crystals, often but dimly seen through 

 thin fragments of the tissue in water, occasionally in the dry state 

 become plainly visible when treated with turpentine or oil of cloves. 

 Other means of detaching the crystals from, or exposing them in, 

 their seat will, of course, be tried by the practical phytotomist ; and 

 to this end boiling in water or in a strong solution of caustic potass 

 will frequently prove more or less efficacious. The alkali sometimes 

 facilitates the separation or isolation of the tissues or cells, so as to 

 show them very advantageously. For example, by this treatment 

 two layers may be plainly demonstrated in the seed-coat of Tamus ; 

 one layer composed of parallel fibres about g- o V o^th of an inch in 

 diameter ; and another layer of roundish or polygonal cells, each 

 containing one of the crystals (Plate XLIV., Fig. 7). And of these 

 may be made and easily preserved novel and beautiful microscopic 

 preparations. 



Composition of the Crystals. — They appear to consist chiefly of 

 oxalate of lime. In none of many trials did the crystals dissolve 

 with effervescence in acids ; though the carbonate of that earth is 

 not uncommon as the main constituent of plant-crystals, as I infer 

 from the experiments I have made on the rhombohedral or some 

 such forms in Cactaceae, and on the sphaeraphides which are often 

 either abundant or deficient in the leaves of Urtica. It is 

 well known that the leaves of Bryonia are studded with scabrous 

 tubercles, and to be regretted that of the intimate nature of these 

 no information is given in the books of descriptive botany. Each 

 of these " asperities " or " callous points " is about xxT^h of an inch 

 in diameter, and composed of many smooth, hyaline, round, or oval 

 granules, the mean size of which is about ^nirth of an inch; and 

 they are soluble with brisk effervescence in dilute acids. 



Form, Size, and Situation of the Crystals. — In the leaves and 

 other parts of Leguminosse the crystals are much of the same size 

 and shape as already described in the testa of rfany other orders. 

 But the figure and size are so variable, even in one plant, as to 

 defy precise and intelligible definition. The mean diameter of the 

 crystals is about jw^th of an inch ; and in the garden pea they run 



