THE STRUCTURAL COMPONENTS OF PROTOPLASTS 



41 



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impressed in later chapters by similarities rather than by (lif'ferences in 

 the fundamental cytological features of the two organic kingdoms. 



Ergastic Matter. — Accumulations of nonproto- 

 plasmic materials in or on the protoplast are called 

 ergastic suhsfances (ergon = work). They are for the 

 most part products of the protoplasm's metabolic 

 work. They may represent reserve matcnials later 

 used as sources of energy in further work, or the 

 useless by-products of such activity, or supporting 

 structures that render bodies of certain t,ypes pos- 

 sible. Such ergastic matter may occur in an^^ part 

 of the protoplast, although it is rarely observed in 

 nuclei. The cell sap, described in a previous section, 

 may be regarded as a mass of ergastic materials in 

 an aqueous medium. 



The most conspicuous ergastic substances in 

 plants are carbohydrates, starch and cellulose being 

 the representatives of this group most often observed 

 in tissues. The starch elaborated in the chloroplast 

 appears as visible granules, and when deposited by 

 amyloplasts in storage organs the granules may 

 become very large. Researches on the structure of 

 such granules have shown them to consist of 

 numerous concentric layers which have been de- 

 posited successively about a hilum, the point at which 

 deposition began (Fig. 31). Unequal deposition on 

 the various sides results in a granule of eccentric form 

 and structure. Compound grains with more than 

 one hilum are plentiful in some tissues. 



The layering in the starch granule has been coi- 

 related with periodic activity of the plastic! caused 

 by the alternation of day and night. Each laj-er 

 consists of CeHioOs units arranged in a regular "space 

 lattice": the granule is a spherocrystal. When such 

 granules are examined in polarized light between 

 crossed Nicol prisms, they present a characteristic 

 and beautiful appearance, each of them being a bright 

 body traversed by a dark cross with its arms meeting 

 at the hilum (Fig. 32). It is sometimes possible to 

 identify small granules in this way when other tests fail. 



Cellulose is the chief material in plant cell walls. Like starch, it 

 consists of CeHioOa units (anhydrous glucose residues) arranged in the 

 form of a space lattice, the layers here being i-elatively flat. Associated 



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Vic. :M). Xolinal 

 and centrifuged eells 

 of V)ean root. In the 

 centrifuged cell (be- 

 low) the centrifugal 

 end is directed down- 

 ward. In order of 

 relative and decI■ea^^- 

 ing .specific gravity 

 the nonnuclear com- 

 ponents are (1) .starch 

 granules and pla.«tids 

 (when present), (2) 

 chondriosomes and 

 proplastids, (.3) cyto- 

 plasm, (4) osmiophilic 

 platelets, (5) cell sap, 

 (()) lipide material. 

 {After H. W. Beams 

 and R. L. Kirifj.) 



