14 PROTOPLASM AND PLANT CELLS 



due apparently to the successive deposition of denser and 

 less dense la3'ers. 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, 



