256 



HALF-HOURS IN THE GREEN LANEb. 



tnin glass cover, and viewed with a quarter-inch 

 obiective. He has shown satisfactorily that these 

 plant crystals are not diseased growths, but natural 

 products, and he expressed his belief that they may 

 ultimately prove useful, as characters, in systematic 

 botany. In many cases, these crystals assume forms 

 which are distinctly geometrical, as in Fig. 194. 

 These prisms are well adapted for the polarisation of 



Fig. 194. 



Crystal Prisms, highly magnified : </, from Quillaja saponaria ; A, from 

 the Testa of Milk Thistle (Silybum marianum) ; , from the Ovary- 

 coat of Spear Thistle (Cardwis lanceolatus). 



light. Generally speaking their chemical composition 

 is oxalate of lime, though some of them appear to be 

 composed of phosphate of lime. The Onion and 

 common Rhubarb are good plants for the young 

 microscopist to experiment upon in search of these 

 interesting objects ; and, as Professor Gulliver has re- 

 marked, it is an occupation fit for the drawing-room, 

 and one well calculated to employ the fair and dextrous 

 hands of ladies. In no plants are the geometrically 

 shaped crystals better seen than in the Knap- weed 



