170 MINUTE STRUCTURE OF THE FLOWER. 



473. The flbro-vascular bundles of the flower are essentially 

 the same as the collateral bundles found in ordinary green 

 leaves, except that their elements are usually more delicate in 

 texture, and in the inner whorls of organs very much reduced. 



474. The parenchyma calls for no special remark beyond allu- 

 sion to the fact that some one of the different kinds of internal 

 glands is frequently associated with it. 



475. The epidermis has stomata, which are generally rudi- 

 mentary, - and most of the forms of trichomes. One of the most 

 interesting peculiarities of structure presented by the parts of 

 the flower is found in the papillar outgrowths alluded to in 222. 

 These are of course minute and short hairs, which, owing to 

 their abundance, impart a velvety appearance to the part on 

 which they occur. This appearance is well shown by the petals 

 of a very large number of the flowers most common in cultiva- 

 tion. 



476. The cuticle of the epidermal cells of the more delicate 

 petals is sometimes very distinctly striated in an irregular man- 

 ner. The walls of the cells generally have a sinuous outline. 



477. The colors of petals and other colored parts of the 

 flower are dependent either on the presence of corpuscles 

 (the colored plastids) or of matters dissolved in the cell-sap. 

 The following account of the coloring-matters in the very com- 

 mon Viola tricolor is condensed from Strasburger. 



A vertical section through a petal exhibits the epidermis of the 

 upper side as consisting of elongated papillae, while that of the 

 lower side has only slightly rounded ones. Just below the epi- 

 dermis of the upper side there is a layer of compact cells, under 

 which are several rows of smaller cells with conspicuous inter- 

 cellular spaces. The cells of the epidermis of both sides contain 

 violet sap and yellow granules ; the layer of compact cells under 

 the epidermis of the upper side contains only yellow granules. 

 The striking diversities in color presented by different parts of a 

 given petal depend wholly upon combinations of these two ele- 

 ments of color ; namely, violet sap and yellow granules. In 

 some places which are devoid of either of these elements there 

 are white spots ; at these places the light is refracted and re- 

 flected by the intercellular spaces which contain air. If the air 

 is removed by pressure, the spots will become transparent. 



478. The cell-sap in the parts of the flower may have almost 

 any color, especially shades of red and blue ; from this sap the 

 coloring-matter sometimes crystallizes in the form of short and 

 slender needles ; for instance, in Delphinium Consolida. 



