144 



A TEXT-BOOK OF BOTANY. 



ture. They vary in shape from ovoid or spherical to polygonal, 

 and have a more or less distinct marking known as the " hilnm," 

 " nucleus," or the POINT OF ORIGIN OF GROWTH. The substances 

 of which the grains are composed are arranged in concentric 

 layers or lamellae which are more or less characteristic and which 

 sometimes become more distinct on the application of certain 

 reagents (Fig. 90). The point of origin of growth and alternate 

 lamellae are stained by the use of gentian violet and other aniline 

 dyes, which may be taken to indicate that these layers contain a 

 colloidal substance somewhat resembling a mucilage, while the 



FIG. 90. Successive stages in the swelling and disintegration of starch grains in the 

 presence of water on the application of heat (6o-7o C.),or certain chemicals. Potato 

 starch i-io; wheat starch 11-22. 



alternating layers are stained with dilute iodine solutions and 

 are probably composed of soluble starch, this latter corresponding 

 to the a-amylose of Arthur Meyer or the granulose described 

 by Nageli. The peripheral layer of the grain appears to be a 

 distinct membrane. It is quite elastic, more or less porous, and 

 takes up stains readily. 



While starch grains usually occur singly, they are not infre- 

 quently found in groups of J;wo, three, or four grains, when they 

 are spoken of as two-, three-, or four-compound. In some of the 



