4 THE PROTOPLAST 



IV. Ergastic Substances. — All living cells in plants contain va- 

 riable amounts of "lifeless" materials -svhicli may be collectively 

 designated as crgasiic substances. They include storage products, 

 waste material, or by-products of protoplasmic activity and in 

 elementary botanical texts are often termed "inclusions." One 

 of the most common examples of ergastic substances is the plant 

 vacuome which consists of a dilute aqueous solution of a Avide 

 range of inorganic as well as organic materials. 



In addition to the vacuome, many types of plant cells contain 

 further ergastic material in the form of reserve food such as 

 starch, proteins and fats or oils. Perhaps the most common type 

 of non-transitory food is starch, Avhicli occurs as grains, the size 

 and form of which are highly specific. Obtain a thin, transverse 

 section of the stem of PcUionia and after mounting it in water, 

 examine the large parenchyma cells of the cortex under law and 

 high magnification. Most of these cells contain starch grains 

 which have developed within the chloroplasts. Often, "frag- 

 ments" of the chloroplast may be seen at the broad end of the 

 pear-shaped starch grains. The addition of dilute iodine to the 

 section will give the blue color reaction typical for starch. In 

 the tissue of such storage organs as tubers, fleshy roots and 

 cotyledons, starch grains ai-e formed by the activity of amifJo- 

 plasts. These starch-forming i)lastids hick chlorophyll and are 

 to be regarded as a specialized type of hucoplasf. Secure a small 

 amount of fresh potato tissue and after gently teasing it with 

 dissecting needles, mount it in water, jidd diluto iodine and ex- 

 amine under low magnification. Note the numerous obovoid starch 

 grains in every cell. Plxamine a single starch gi-ain under high 

 magnification and observe, near the smaller end of the grain, the 

 minute refi'active i)()iut Avhicli is termed the hit inn. Careful regu- 

 lation of the light and patient use of the fine adjustment will 

 usually reveal a nninbci- of more or less distinct rcc< )ityic lagers 

 arranged abdut the hilniu. For coniiiai-ative pui'poses, examine 

 the storage parenchyma cells in the ])can cotyledon, noting the 

 difference in the foi-ni and [lositidii of the hiluin in tlie ovoid 

 starch grains. 



A very common type of ergastic su])stance in many kinds of 

 ])lant cells is calcium oxalate which api)ears usually in the form 



