14 PROTOPLASM AND PLANT CELLS 
due apparently to the successive deposition of denser and 
less dense layers. At first the grains are entirely en- 
closed by the plastid but as they increase in size they 
become excentrically located and seem eventually to 
burst out of the plastid at one side. In the chloroplasts 
containing pyrenoids the starch grains are mostly pro- 
duced in intimate connection with the latter. 
24. Aleuron. In the dry seeds of many plants there 
may be found, sometimes in a definite layer of cells, 
sometimes scattered throughout the cells of the seed, 
small rounded or frequently angular granules of a protein 
substance called aleuron. ‘This is stored up in the cells 
as food for the young seedling. ‘These aleuron grains are 
formed in small vacuoles in the cytoplasm, the aleuron 
being in solution at first but appearing as granules or 
even crystalloids as the seed loses its moisture in the 
process of ripening. As the seed absorbs water prepara- 
tory to germinating the aleuron goes into solution again 
and is used up for food. Aleuron is frequently found in 
cells containing other stored up food matter such as 
starch or oil. It was formerly supposed to be a dry 
stage of protoplasm but is now recognized as one of the 
highly complex food substances out of which protoplasm 
can be formed by the cell. 
25. Oils or Fats. Many plants provide for the use of 
the young seedling a supply of fat instead of starch. 
This is usually present in the cell as very minute drops, 
in fact almost as an emulsion throughout the cytoplasm. 
Sometimes the oil droplets are of considerable size, in 
very oily seeds often filling all the interstices of the cyto- 
plasm. Usually these fats are liquid but in some plants 
they are semisolids of the consistency of butter. They 
are mostly true fats, similar to those found in animals, 
