RUST, SMUT, ETC. 115 



mycelium is developed either under ground, or in the interior 

 of the substance on which the plant grows. The filaments of 

 the mycelium are composed of elongated colourless cells. Fungi 

 are propagated by seeds or sporules enclosed in sporule cases or 

 spores (b, c, fig. II.) 



20 1 . Fungi most commonly grow upon vegetable or animal 

 substances in a state of decomposition. They require a very 

 large supply of carbonic acid and ammonia for their nutrition. 

 The proportion of nitrogenous matter contained in their tissues 

 is much greater than in those of any other vegetable ; so that 

 their substance, if capable of being digested, is almost as nutri- 

 tious as flesh. (*) 



202-3. All cultivated plants are covered with a membrane, 

 termed the cuticle, and composed of cellular tissue (fig. I, p. 

 113.) The cells of the cuticle are filled with a colourless fluid, 

 and their walls are thickened on the outside with a deposit which 

 is usually of a waxy nature and nearly impervious to moisture. 

 In plants growing in temperate climates, the cuticle is composed 

 of a single row of thin-sided cells, in tropical plants several layers 

 of thin-sided cells occur, evidently with a view to resist, by their 

 non-conducting power, the great heat of a tropical sun. Exter- 

 nally to the cuticle, there is an exceedingly delicate transparent 

 membrane called the epidermis. 



204. In particular parts of the cuticle of nearly all plants, 

 minute openings exist which are termed stomata ; these may be 

 opened or closed by an alteration in their form. They are not 

 found upon the roots of plants, on the ribs of the leaves, or in 

 plants growing in darkness, but they exist in general on all leafy 

 expansions. They are most abundant on the under surface of 

 leaves, except when these float on water, and then they are found 



(1) Carpenter. Prin. of Comp. Physiology. 



