MORPHOLOGY OF HIGHER PLANTS. 



163 



to show that it is in the nature of a sphero-crystalloid,- resembUng 

 inuhn in some respects. Starch grains have an interesting struc- 

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

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

 " 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 



Fig. 96. 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 II-.22. 



reagents (Figs. 96, 97). The point of origin of growth and alter- 

 nate 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 alternating layers are stained with dilute iodine solu- 

 tions and arc probably composed of soluble starch, this latter cor- 

 responding 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. 



