22 VEGETABLE ELEMENTS AND TISSUES. 



appears to be the case to the naked eye, but from the 

 presence of little grains or granules of a green colour- 

 ing-matter in the protoplasm of the cells. This green 

 matter is called chlorophyll. If the cells be crushed, 

 the granules will escape, and can be examined in the 

 separate state. Chlorophyll is most abundant in 

 those parts of plants which are exposed to the light. 



Starch. In many cells of plants, particularly those 

 which have attained their full growth, other granules, 

 larger than those of chlorophyll, and colourless, are 

 met with ; these are the starch- granules (PL I. fig. 3). 

 They are usually rounded or oblong, and exhibit on 

 the surface a number of rings, one within the other, 

 or concentric, as it is called. In the centre of the in- 

 nermost ring is a black dot or streak, arising from 

 the presence of a little pit or furrow, and called the 

 hilum. 



The starch- grains may be readily seen within cells 

 in a thin section of a potato (PL I. fig. 4) ; here they 

 are very numerous, and larger than in most other 

 plants. A separate grain is represented in fig. 3. 



The appearance of rings in the separate grains 

 arises from the starch -granules being composed of 

 numerous concentric coats or layers, like those of an 

 onion. 



A very simple and striking method of determining 

 whether any granule is composed of starch or not, 

 consists in adding to it, when placed in water on a 

 slide, a drop of solution of iodine. As soon as this 

 touches the granule, it assumes a beautiful purple 

 . colour, the depth of tint depending upon the quantity 

 of the iodine-solution ; if this be very considerable, 

 the granule appears almost black. The section of 

 potato forms a very interesting object when mois- 

 tened with the iodine- solution, the starch-granules 

 becoming beautifully coloured, whilst the cell-wall re- 

 mains colourless, and the protoplasm becomes yellow. 



The form of the starch-granules differs in differ- 



