CONTENTS OF CELLS. 



501 



The nature, mode of formation, and chemical history of this sub- 

 stance all stand in need of further investigation. 



Crystalloids. Masses of proteinaceous substance of a crystalline 

 form, but differing chemically from true crystals, occur imbedded 

 in the protoplasm of many plants, especially of such as are in a 

 dormant condition, as beneath the rind of the tuber of the Potato. 

 Their function seems to be to act as a reserve of nourishment to 

 be used when growth becomes active. Similar crystalloids have 

 been seen in many Red Seaweeds. 



Starch. Starch-granules appear to occur throughout every class 

 of plants except the Fungi, and are perhaps most frequently of 

 globular form when young : but when they acquire any consider- 

 able size their form usually diverges from this, and presents very- 

 remarkable varieties, often attributable to the conditions in which 

 they grow. Full-grown starch-granules are not homogeneous, but 

 marked with striae indicating the concentric laminae of which they 

 are composed. These laminae are alternately of denser and softer 

 consistence, and surround a commonly more or less excentric point, 

 usually of very small size (fig. 550), which often appears solid when 

 the starch-granule is fresh, but which forms a minute cavity, fre- 

 quently running out into a few radiating cracks, when the starch- 

 granules are dry. 



The granules occur either singly or collected in masses of definite 

 shape, forming compound granules (fig. 551) very often they exist 



Fig. 550. 



Fig. 551. 





Fig. 550. Starch-granules of Potato. Magn. 400 diam. 



Fig. 551. Compound starch-granules: a, a double granule from the Potato; 6, grouped 



granules and two fragments from the rhizome of Arum maculatum. Magn. 



400 diam. 



in the interior of chlorophyll-corpuscles or bands, or imbedded in 

 the protoplasm lining the cell-wall. In certain tissues they fill the 

 cavity of the full-grown cell, and in some cases so densely that they 

 become moulded into polygonal forms by mutual pressure. 



