THE WALLFLOWER 107 



an ordinary foliage-leaf, but with simpler ramifications. 

 In each bundle the xylem and phloem are in the 

 usual position, the xylem being placed on the upper 

 side, i.e. on the side turned towards the middle of the 

 flower, and the phloem on the lower or outer side. 

 The mesophyll is of uniform parenchymatous structure 

 throughout, showing no differentiation between palisade 

 and spongy tissue. The cells contain numerous 

 plastids corresponding to the chlorophyll granules, 

 but little or no chlorophyll is present. The dark 

 colour of the sepal is due to purple cell-sap, which is 

 present in the more superficial layers on both the 

 upper and lower sides of the sepal. It is most 

 abundant towards the lower surface, and especially in 

 the hypodermal layer, though it also occurs in the 

 epidermis. Stomata are fairly numerous on both 

 surfaces of the sepal. Hairs of the structure already 

 described and figured (see Figs. 26 and 27) are pro- 

 duced on the lower or outer surface only. There is 

 reason to believe that they serve to protect the young 

 flower-bud from the attacks of slugs. 



The petals, which form the next floral whorl, are in 

 many respects of very different structure. The petals 

 are at first very slow in their development, and 

 become mature rather suddenly, shortly before the 

 flower is ready to open. They are transitory struc- 

 tures ; they have no protective function, nor do they 

 take any direct part in fertilisation or the production 

 of the embryo, but they are none the less important. 

 The Wallflower is in a great degree dependent on the 

 visits of insects for its fertilisation. The function of 



