108 STRUCTURAL BOTANY 



the petals is to render the flower conspicuous, and 

 thus to help in attracting insect visitors. Hence, it 

 is generally the corolla which constitutes the chief 

 beauty of the flower in our own eyes. Its business is 

 to be showy. Each of the petals has a vascular 

 system; its bundles, though slender, are of normal 

 structure, with xylem above and phloem below. The 

 mesophyll consists entirely of spongy parenchyma 

 without chlorophyll. The epidermal cells of both 

 surfaces contain very numerous yellow chromoplastids 

 that is, protoplasmic bodies like chlorophyll granules, 

 but containing a yellow pigment instead of chloro- 

 phyll To these the yellow colour of the petals is 

 due. In cultivated Wallflowers a purple cell-sap is 

 also present, chiefly in the lower epidermis. To the 

 combination of the colours of the yellow chromo- 

 plastids and of the purple cell-sap the flower owes 

 those beautiful red-brown shades which we know so 

 well. The wild Wallflower generally has the chromo- 

 plastids only, and so its petals are simply yellow. 



There are no stomata on either surface of the petal. 

 The epidermis is not smooth, but its cells all grow 

 out into short hairs or papillae, which are narrower 

 and more prominent on the upper surface of the 

 petal. They give it a peculiar velvety gloss, owing 

 to the varying way in which they reflect the light, 

 and this effect is perhaps enhanced by the fact that 

 the cell-walls of the papillae are delicately ridged. 



The well-known pleasant scent of the Wallflower 

 comes from the corolla. This scent is probably of 

 importance as helping to attract insect guests. 



