THE CELL. 



17 



The two first named substances (starch-grains and aleuron-grains) 

 are represented highly magnified in the figures shown below. Fig. 6 

 represents both starch and aleuron as they occur in seeds of Pisum 

 sativum (pea). Fig. 7 represents starch-grains from a potato-tuber. 



(a) Starch. 



In 1858 NAGELI made known the results of his investigations 

 relative to the growth of substances capable of imbibition (as opposed 

 to crystals), especially starch-grains and cell-membranes. The chief 

 conclusion arrived at is that growth of starch-grains and cell-walls is 

 ~by intussusception and not by apposition as in crystals. (The cell- 

 wall will be discussed later.) 



Stratification of starch-grains is not the result of deposits of 

 successive layers so that the outermost layer is the youngest. On 



FIG. 6. Cells from the seed of Pisum 



sativum. 

 (X 800.) (After Sachs.) 



FIG. 7. 



A, simple starch-grain of potato-tuber; 

 5, partial compound ; C, compound. (X 800.) 



the contrary the layers are the result of internal processes of growth 

 arid differentiation : there is nothing superimposed upon the outer- 

 most layer, which really existed from the very beginning. The 

 cause of the stratification is to be found in the alternating layers of 

 greater and lesser percentage of water (therefore more and less dense 

 layers); since excessive evaporation or absorption of water causes 

 them to become less distinct. Optically, by the aid of the polariza- 

 tion microscope, it can be shown that starch-grains react as though 

 composed of uniaxial crystals. Under the crossed Nicol prisms 



