THE LEAVES. 13 ' 



358). These scales may represent the sheathing base only ; 

 they may be the base with the stipules (fig. 139); or they 

 may represent the leaf base and the blade. The petiole in 

 all cases is wanting. In addition to the modification of 

 form, scales, especially those that are protective, have their 

 tissues firmer and more resistant to cold and unfavorable 

 external conditions. Not infrequently the scales are covered 

 with secreting hairs, or possess glands sunk beneath their 

 surfaces, whose function is to produce and excrete resins and 

 similar materials. The inner protective scales of buds (fig. 

 98) are often covered with an abundant coating of woolly 

 hairs, which serve to prevent rapid change of temperature in 

 the interior of the bud. 



161. (4) Flower leaves and bracts. — -On certain parts of 

 the stem, leaves are commonly profoundly modified to carry 

 the spore cases. They are called sporophylls (c, si, fig. 104). 

 (See ^[ 329.) Adjacent to these are others which may be 

 highly colored and adapted in form to protect the sporo- 

 phylls, and to facilitate the visits of insects (s, p, fig. 104). 

 A shoot whose leaves are thus clustered and specialized con- 

 stitutes a " flower." The leaves adjacent to the flower leaves 

 are also more or less modified in form and reduced in size. 

 They are called bracts (// •> 2 > 3. fig. 139). (See also ^[ 359.) 



162. (5) Storage leaves. — Other leaves are utilized for 

 purposes of storage. For this purpose the ribs are reduced in 

 number and size, while the softer tissues of the leaf are often 

 enormously developed, and serve as the receptacles of the 

 reserve food. The primary leaves of the seed plants (coty- 

 ledons) are often much distorted by the deposit in them of 

 reserve food for the embryo. When such leaves possess 

 sheathing bases the structure resulting from the union of a 

 number of such leaves upon a short axis is called a bulb. 

 (See also ^f 109.) The leaves of buds are sometimes thick: 

 ened by the deposit of food material, and when such buds 



