MORPHOLOGY OF HIGHER PLANTS. 171 



So-called White Colors. — The so-called white colors in 

 plants do not properly belong to either class, but may be said to 

 be appearances due rather to the absence of color, and depending 

 upon the reflection of light from transparent cells separated by 

 relatively large intercellular spaces containing air. In other words 

 the effect produced by these cells may be likened to that pro- 

 duced by the globules in an emulsion. The white appearance is 

 most pronounced in the pith cells of certain stems, where on 

 the death of the cells the size of the intercellular spaces is in- 

 creased and the colorless bodies in the cells as well as the walls 

 reflect the light like snow crystals. 



Calcium oxalate is found in many of the higher plants, and 

 in the algae and fungi as well ; while in the mosses, ferns, grasses 

 and sedges it is seldom found. It occurs in plants in crystals of 

 either the monoclinic or tetragonal system (Figs. 281, 282). The 

 crystals dissolve in any of the mineral acids without effervescence 

 and their identity is usually confirmed by the use of dilute hydro- 

 chloric acid. The crystals of the monoclinic system are rather 

 widely distributed, while those of the tetragonal system are less 

 frequent in their occurrence, being found in species of Allium, 

 Tradescantia and Begonia, in Paulownia imperialis and in the 

 Cactaceas. The crystals belonging to the monoclinic system in- 

 clude a number of forms, as follows : ( i ) Rosette aggregates, or 

 what are commonly termed rosette-shaped crystals; (2) prisms, 

 pyramids and elongated or irregular hexagonal-shaped crystals ; 

 (3) crystal-fibers; (4) raphides; (5) cryptocrystalline crystals 

 and (6) membrane crystals. 



Rosette aggregates of calcium oxalate consist of numerous 

 small prisms and pyramids, or hemihedral crystals more or less 

 regularly arranged around a central axis, and have the appear- 

 ance of a rosette or star (Fig. 281, A). The development of 

 these aggregates may be readily observed in the stem of Datura 

 stramonium. Crystals of this class are more widely distributed 

 than any of the others, and are found in a number of drugs. 

 (See chapter on Powdered Drugs.) 



Monoclinic prisms and pyramids are also widely distrib- 

 uted and are frequently so modified in form that they are of an 

 elongated or irregular hexagonal shape (Fig. 281, C, E). The 



