22 A HALF-HOUR WITH THE 



The external forms of the tissues of plants 

 having been examined, we are now prepared to 

 regard their contents. In the interior of the 

 cells forming the roots and the growing parts of 

 plants, will be observed a number of minute grains, 

 generally of a roundish form. If we make a thin 

 slice of a potato, these granules may be very 

 obviously seen, lying in the interior of the cells of 

 which the potato is composed (Fig. 59, PI. 3). If 

 we now take a drop of the solution of iodine, and 

 apply it to these cells full of granular contents, we 

 shall find that the granules assume a deep-blue 

 colour. This is the proof that they are starch ; and 

 as far as we at present know, no other substance 

 but starch has the power of assuming this beautiful 

 blue colour under the influence of iodine. We 

 have thus a ready means at all times of distinguish- 

 ing starch. The grains of starch are of various 

 sizes and shapes. The starch of the flour of wheat 

 has a round form, and varies in size (Fig. 57, 

 PL 3) ; that of the oat is characterized by the 

 small granules of starch adhering together (Fig. 58, 

 PL '3) in globular shapes. When these globules 

 are broken up, the grains appear very irregular. 

 In the arrow-root called " Tous les Mois" the 

 grains are very large (Fig. 60, PL 3), and, like those 

 of the potato, they look as if composed of a series of 

 plates laid one upon the other, gradually becoming 

 smaller to the top. These lines do not, however, 

 indicate a series of plates, but rather a series of 

 contractions of a hollow vesicle or bag. This 

 vesicular character of starch may be made apparent 

 by gently heating it, after moistening, over a spirit- 

 lamp on a glass slide, or by dropping on it a drop 

 of strong sulphuric acid. Sago (Fig. 62, PL 3) 

 and tapioca (Fig. 63, PL 3) are almost entirely 

 composed of starch, and may be easily examined 



