BOTANV OF THE LIVING PLANT 



. i it a common polysaccharide in plants, and is built up from many 



molecules. LI forma ubstance, either of a 



in fee chloroplasta oi the leal where its | e in 



„ Ulth p i a noted: or of a more permanent 



IMt . many kinds. It ia almosl always deposited 



in the form ol grains within plastids, either the « nlortplasts of green organs, 



,, r . tponding but colourless Uucoplasts ol uon-green organs (Figs. 



j 1( g, | lh,- presence within plant tissues oi an enzyme, Diastase, 



TQB. 



Fig. 8i. 

 inn Potato. A, with minute leucoplasts surrounding the nucleus. 

 B already fanning starch, stained darkly with iodine. C shows 



furt irmation, and one cubical protein crystal. ( ■ 220.) 



which promotes the decomposition of starch into maltose, can easily be 

 demonstrated, and it ia generally assumed that the same enzyme at other 

 times builds up starch from maltose. This enzyme appears to be located 

 in the pi. 1st ids. Starch grains show a characteristic stratification, which is 

 related to their growth by apposition (Fig. 82), while the blue-black coloration 

 with iodine is also distinctive. They may be simple or compound, and are 

 larger in storage organs than in the leaf. 



Inulni i> another reserve polysaccharide, built up from fructose, and differ- 

 farther from starch in its restricted occurrence in plants, and by its exist- 

 in .1 state of colloidal solution in the cell-sap. It is present in large 

 quantity in the storage organs of members of the Compositae, e.g. Dahlia 

 roots and Artichoke tubers. 



Uulose is the substance from which the walls of plant-cells, especially 

 of young cells, are largely built up, and is thus of structural rather than of 

 nutrition. d importance. The cellulose is deposited by the protoplast. Its 

 molecule ia larger than that of starch, but it again is built up from a large 

 number of glucose mole ules, presumably under the influence of enzymes. 

 An enzymi m 1 apable of converting cellulose to glucose has been detected 



in a few of the lower plants. Cellulose ia a colloidal substance with a strong 

 attr.u tion for water ; hence the cell-wall usually contains a high proportion 

 of imbibed water. 1 the original cellulose may become impregnated with 



other substances Mich as Lignin (in xylem, and particularly in fibrous ele- 

 ments), giving increased mechanical strength : or Cutin and Suberin, in 

 lermal and cork cells, to which they give impermeability to water (see 



