86 



BOTANY FOR BEGINNERS 



CHAP. 



they are always undergoing a change due to the action of a 

 ferment found in the cells. This ferment, which is called 

 diastase, reconverts the starch into sugar. Large starch grains 

 are only found in those parts of plants where they are stored 

 up for future use. Each plant produces a starch_grain which 

 .differs in shape and size from the grains produced by other 

 plants. By making use of this fact, adulterations of foods can 

 be detected under the microscope. 



Starch grains are always striated. The organic centre of the 

 grain around which it grows by the deposition of new material 

 is termed the hilum. The hilum is pro- 

 duced by the activity of either the chloro- 

 plast or the leucoplast, and the successive 

 layers which are deposited are also due to 

 the same activity. Starch grains grow in 

 the same way that a cell-wall grows in 

 thickness, that is, by apposition or by 

 intussusception. In some cases compound 

 grains may be found in cells. These can be 

 divided into two kinds, (a) those called 

 spurious, which are produced by two or 

 more grains coming together and uniting 

 as a result of pressure, (b) true compound 

 grains which are produced by the same 

 leucoplast, and round which there are 

 always a number of layers which bind the 

 grains together. 



Starch grains can always be detected in 



cells by treating them with iodine solution, when they give a 

 deep-blue colour. They thus differ from cellulose which only 

 gives a yellow colour with iodine. 



Starch is a carbohydrate having the same composition as 

 cellulose but differing in its physical properties. When treated 

 with potash solution starch swells up, and if boiled with 

 water will form a paste. If heated while dry, starch is converted 

 into dextrine and becomes soluble. 



Aleurone Grains. Aleurone grains, or as they are some- 

 times called, proteid grains, are found in many seeds. Each 

 aleurone grain Is built up of a crystalloid and a globoid. 

 The crystalloid is composed of albumen or proteids, and a 



FIG. 112. Starch 

 grains from seed- 

 leaves of Bean. 

 (X 540.) (S.) 



