VEGETABLE SECRETIONS. 



41 



Raphides. Another secretion found very abundantly in plants is certain crystal- 

 line bodies termed Raphides, from the resemblance of some of them to a needle 

 (raphis). The term, however, is not a happy one ; since many varieties of these 

 crystals exist which have no resemblance to a needle. They are not secreted in the 

 form in which we see them, but are deposited from the secretions. They occupy 

 both the cavities of the tissues and the passages which lie between the tissues, but 

 are the most abundant in the cells of succulent plants. 

 They may be observed with great ease in the stem of the 

 common garden rhubarb (Rheum), or of the balsam, and 

 in the bulbs of the onion, and all bulbous garden plants. In 

 the former case they have a square outline, and are isolated 

 (Fig. 87), or they are aggregated into separate star-like 

 bodies (Fig. 88) ; whilst in the latter they are usually needle- 

 form, and lie in dense bundles (Fig. 86) . Their number is so 

 great as to impart a grittiness to rhubarb-root when bitten ; 

 and the most so in the finest specimens of Turkey rhubarb. 

 Their chemical composition is that of oxalate, phosphate, 

 tartrate, malate, or citrate of lime, and in size they 

 differ from one-fortieth to one-thousandth of an inch. 

 Phosphate of lime is found abundantly in the bones of 

 the animal body, but not in the precise form in which we 

 observe it in Raphides. "We have no instance of oxalate 

 of lime crystals in the body ; but they are not unfre- 

 Fig. 86. Rhapides ; acicular quently met with in the urine of persons, both in apparent 

 LXn^VebS tKquili health and in disease ; so that it has been inferred that 

 (Scilla mauritanica). ft^y have b een introduced with the food. 



We do not know the uses of these substances in the vegetable economy; but 

 although they render certain plants brittle, it 

 is not ascertained that they are the result of any 

 diseased action. This brittleness is the best seen 

 in some of the large Cactus plants (Fig. 89). 

 One which was re- 

 moved, after a lapse 

 of a thousand years, 

 from the woods of 

 South America to the 



Fig. 88. 



Royal Gardens at 

 Kew, was wrapped in 



Fig. 87. Raphides found in the common 

 onion (Allium). 



A, octohedral. B, prismatic. 



C, a stellate or star-like cotton, and packed as 



mass of crystals found in though it were the 



rhubarb root. 



most fragile of sub- 

 stances. They are readily seen on microscopic examination, if a thin section of an 

 onion be placed in water in the usual way ; but as they are found in all parts of a 

 plant, from the rough bark (Fig. 90) to the delicate spiral vessels and the pollen, they 

 will be observed in almost every investigation. 



They have beea produced artificially, and, so far as may be seen, in a state as 

 perfect as those deposited from the vegetable juices. The late eminent botanist, the 

 brother of Professor Quekett, produced the stellate and rhomboihedral forms artificially 



