21 A HALF-HOUR WITH THE 



several of these crystals unite together around a 

 central mass, forming a stellate body. These bodies 

 have been called "crystal glands," but they have 

 no glandular properties. They may be seen in 

 the root and leaf- stalk of common rhubarb (Fig. 

 66, PL 3), and may be easily observed in a bit 

 of rhubarb from a spring tart. These crystals 

 are mostly formed of oxalate of lime. They are 

 constantly found in plants producing oxalic acid. 

 The gritty nature of rhubarb-root arises from 

 the presence of oxalate of lime. Sometimes the 

 oxalate of lime assumes a round disk-like form. 

 Snch forms are seen in plants belonging to the 

 cactus family (Fig. 68, PL 3). 



Other substances, besides oxalate of lime, are 

 found crystallized in the interior and on the surface 

 of plants. Crystals of sulphate of lime have been 

 found in the interior of cycadaceous plants. Car- 

 bonate of lime is found in crystals on the surface of 

 some species of Char a, or stone wort. There is a 

 shrub not uncommon in gardens, known by the 

 name of Deutzia scabra, on the under surface of 

 the leaves of which there are beautiful stellate 

 crystals of silica. The best way of seeing these is 

 to put the leaf under the Microscope, and to examine 

 it by the aid of reflected light. Sugar and honey 

 also assume a crystalline form, and may be known 

 by the shape of their crystals (Figs. 228, 229, 230, 

 PL 8). 



The external surface of the parts of all plants 

 will afford a rich field of amusement and instruction 

 to the microscopic observer. The cuticle, or epi- 

 dermis, of which we have before spoken, has a very 

 varied structure, and contains the little openings, 

 stomates, before described. The cuticle, which, in a 

 large number of cases, is smooth, becomes elevated 

 in some instances, and forms a series of projections, 



