Metaphyta L ilium. 



155 



hairs may become matted together, giving origin to a prickle, 

 as in the hop, whilst in the nettle the leaf is covered with 

 multicellular hairs, with the contents of the terminal cells 

 modified into a substance capable of producing considerable 

 irritation should its end be broken off in the skin. These 

 may be termed glandular hairs. 



Before passing to the consideration of the sporophylla, 

 we must glance at the nature and mode of origin of certain 

 substances which we have had occasion frequently to men- 

 tion in reference to many points in our past survey, and 

 which may conveniently be discussed at this point. They 

 are substances found in the cells of plants, such as starch, 

 inulin, aleurone, crystalloids, crystals, and such-like ; chloro- 

 phyll we have already discussed (p. 38). 



FIG. 73. STARCH GRANULES OF Solatium tuberosum. (Thom6.) 



Starch. Starch is one of the most important of the 

 derivatives of plant protoplasm. It is a compound of 

 extreme abundance, some parts of plants, such as the potato 

 tuber, consisting almost entirely of starch. Starch occurs 

 in the form of granules, which possess a characteristic form 

 for different plants. We may take those of the potato as 

 a type. Each granule is ovoid in shape, and presents a 

 usually eccentrically placed point known as the hilum 

 round which the starch is arranged in a number of layers, 

 alternating with thin films of water. The hilum is the 



