216 BRITISH FLOWERING PLANTS chap. 



important to the leaves to secure as much light and 

 air as possible, and when growing on a flat surface the 

 ivy shape enables the leaves to fit into one another, and 

 to cover the whole surface. On the other hand, the 

 flowering branches grow up into the air. The leaves 

 are arranged round the stem, and under these condi- 

 tions an oval form is more suitable, as, for instance, is 

 also seen in the Black Poplar. The structure of the 

 leaf differs according to the amount of light to which it 

 is exposed. In the sun two layers of palisade cells are 

 developed under the upper epidermis, while in the shade 

 leaves the whole interior consists of rounded cells. The 

 difli'erence is very like that which exists in the Dandelion 

 (see p. 250). The ivy is almost, if not quite, the last 

 of our plants to flower. The peduncles are covered with 

 stellate hairs. The five united carpels form a pistil 

 projecting about 1 mm. above the sepals. According to 

 Delpino the flowers are protandrous, while H. Miiller 

 and Kirchner describe them as homogamous. Honey is 

 abundant, and the flowers are very attractive to insects 

 — especially flies. They appear to be sterile to their 

 own pollen. The fruit, which is set during the late 

 autumn, remains almost, if not quite, dormant during 

 the winter, and does not ripen until the following 

 spring. 



LOEANTHACEiE 



Shrubby or half-succulent evergreens, parasitic on 

 trees. 



ViSCUM 



V. album (Mistletoe). — This interesting species is the 

 subject of many folklore tales. The traditionary connec- 

 tion with the Druids is more or less doubtful. Its 

 favourite tree appears to be the Black Poplar. It also 

 does well on the Apple, and some Conifers, as the Silver 

 Fir. It attains an age of forty years. The berry is 

 white, enclosing a single seed, surrounded by a very 



