ERGASTIC SUBSTANCE.S O 



of conspicuous well-defined crystals. It is generally held that 

 such crystals represent an excretory product of the protoplast, 

 being formed by the union of calcium with oxalic acid. The 

 recent monograph by Netolitzky (1929), however, reveals that 

 substances other than oxalic acid may combine with calcium to 

 produce crystalline bodies. Comparatively little is known con- 

 cerning the factors, chemical and biological, which regulate the 

 rate and mode of crystallization, and which hence determine the 

 form of the adult crystal. Netolitzky (1929, p. 47) concludes 

 that in the final analysis it is ''the nucleus which determines 

 which form of crystal will be produced, perhaps by regulating 

 the velocity of crystallization wnthin the cell itself." Examples 

 of the three main forms of plant crystals may now be studied. 



1. Druses or sphaerraphides are compound and consist of 

 more or less spherical aggregates of sharp pointed angular crys- 

 tals, the whole mass often suggesting in form the mace-head of 

 medieval warfare. Examine the prepared section of the stem of 

 geranium {Pelargonium), noting the presence of druses in many 

 of the cortical cells. Transverse sections would suggest that the 

 crystal containing cells are solitary and isolated from one another. 

 But in longisection, it will be seen that frequently the crystal 

 containing cells or crystal sacs (cf. Haberlandt) are in short 

 supposed series, each cell of which may contain a druse. 



2. Raphides are long slender needle-shaped crystals which 

 typically are arranged parallel to one another in definite bundles. 

 Such crystals appear to be most common in the monocotyledons. 

 According to Netolitzky (1929, p. 48) raphides constitute vir- 

 tually a family characteristic in the Oenotheraceae. Obtain a 

 single living plant of duckweed {Lemna sp.) and mount it in 

 water. Note under high magnification that many of the trans- 

 parent cells at the margins of the "thallus lobes" contain promi- 

 nent bundles of raphides. For comparison, examine under lovr 

 magnification freshly-cut longi-sections of the stem of Trades- 

 cant ia noting the much larger raphides, many of which may be 

 pulled from the cells during the process of sectioning. 



3. Prismatic crystals are common in vascular tissue but may 

 also occur singly, or in association wdth other crystal types in 

 thin-walled cortical parenchyma cells (cf. Exercise VI). Ex- 

 amine prepared slides of the stem of Tilia or some similar woody 



