OIL, ALEURONE 



45 



S^- A 



B 



Fig. 19. — Section of a grain 

 of wheat. A, Pericarps and 

 seed coats; B, layer of cells in 

 the endosperm containing 

 aleurone grains; C, cells of the 

 endosperm containing starch 

 grains. 



Prominent among these are the various vegetable oils, which are 

 also compounds of carbon, hydrogen and oxygen. Globules 

 of oil may be found in the tissues of the leaf and in other parts 

 of the plant, but it is especially in the seeds of certain plants 

 that large quantities of oil are stored 

 up, to serve the same purposes that 

 is served by starch in other cases. 



95. Aleurone is a substance which 

 contains, besides carbon, hydrogen 

 and oxygen, also a small per cent, 

 of nitrogen. It is therefore called a 

 nitrogenous substance and is very 

 much like albumen. It is soluble in 

 water and consequently disappears 

 when immersed in a watery solution, 

 but by mounting tissues of dry seeds 

 containing it in a medium like glyc- 

 erine, the aleurone may be seen under the microscope in the 

 form of small granules. 



Differentiation of Tissues 



96. All reserve food materials are ultimately converted into 

 protoplasm and from protoplasm the various structural 

 elements of the tissues are formed. Thus the undifferentiated 

 cell walls of parenchyma consist of cellulose, CeHioOs, a sub- 

 stance having the composition of starch. The cellulose is 

 formed from layers of protoplasm by a process of chemical 

 transformation. 



97. By further alteration in the chemical nature of the cellu- 

 lose walls by which the proportion of carbon is increased, the 

 walls assume special characteristics; the surface wall of epider- 

 mal cells becomes cutinized (cutin), the walls of cork cells be- 

 come suberized (suberin), and the walls of wood and bast 

 fibres become lignified (lignin). 



