152 H. J. Arnott 



Studies of Calcification in Plants 



H. J. Arnott 

 Cell Research Institute, University of Texas, Austin, Texas, U. S. A. 



It has long been known that insoluble calcium salts in the forms of the carbonate, 

 sulfate, and oxalate occur as optically visible crystals in a wide variety of plants 

 (SoLEREDER, 1908). The deposits are commonly considered to be waste products and 

 occur in a variety of shapes, often seeming to fill a cell. Calcium oxalate crystals are 

 by far the most common, and a single cell may contain from one to many hundreds. 

 Crystal cells have been studied by light and polarization microscopy (see reviews in 

 GuiLLiERMOND, 1933; KusTER, 1956) and the crystals have been studied by various 

 techniques including X-ray and infra-red analysis (Pobeguin, 1943; Walter-Levy, 

 1962). 



The salient features of a variety of crystal cells investigated by Arnott and 

 Pautard (1965) are: 1. Crystals are produced within active cells as a part of their 

 program of differentiation, usually in meristematic tissues. 2. Crystal formation is 

 not a random event but takes place in a series of biologically organized steps. 

 3. Crystal development occurs within chambers which first become loaded with an 

 electron dense material and subsequently appear to function as boules in which 

 crystalization occurs, often, but perhaps not always, shaping the crystals. 



A previous study of the growing root tip of Yucca (Arnott, 1962) revealed that 

 raphide cells develop in linear rows with as many as 27 cells in a single file. The 

 present research has shown partly differentiated crystal cells within the zone of active 

 mitosis, in juxtaposition to cells in division and only a few hundred microns from 

 the root tip. In the root zone, where the major growth parameter is cell enlargement, 

 raphide cells are very well differentiated (Fig. 1) and exhibit two characteristics not 

 found in their immediate neighbors. First, they contain a series of electron dense 

 compartments within a crystal vacuole, /. c, the vacuole in which the crystals 

 develop. Secondly, they possess numerous highly differentiated crystaloplastids, /. e., 

 the modified plastids which occur in crystal cells. 



The crystal chambers are elongate, and when cut in transverse section have 

 rectangular or rounded profiles. At an intermediate stage in crystal ontogeny, the 

 chambers exhibit an electron-dense material which does not give an electron diffrac- 

 tion pattern; however, after prolonged heating in the electron beam a pattern 

 suggesting calcium oxide is produced, and at the same time obvious changes occur in 

 the electron-dense material. This dense material is rapidly leached by water in 

 sectioning or by the staining of thin sections with uranyl acetate and/or lead citrate. 

 Therefore, it must be studied in unstained preparations rapidly picked up from the 

 trough. Such data indicates that the chambers are not crystals or crystalline at the 

 electron diffraction level, but are merely loaded with an electron-dense material of an 

 unknown nature. The exact manner in which crystallization occurs is also not under- 

 stood; however, X-ray data (Steinfink et al., 1965) shows calcium oxalate mono- 

 hydrate to be present in the mature crystals. The fact that the mature crystals in 

 Yucca and other plants often have rounded contours rather than sharp facets, means 

 one cannot overlook the possibility that the crystal compartments act as boules, 

 and are in part responsible for the rounded contours found in many plant crystals. 



