142 BRITISH FLOWERING PLANTS chap. 



ACEEACE^ 



Acer 



Of this genus two species occur iu the British Isles, 

 the Sycamore and the Maple. 



A. Pseudo-platanus (Sycamore) has leaves somewhat 

 resembling those of the Plane, whence the name. It 

 is a native of Central Europe, and has no claim to 

 be considered indigenous, though an ancient intro- 

 duction. In autumn the leaves often present a number 

 of black blotches, looking like drops of tar, each about 

 half an inch in diameter. These are due to a parasitic 

 fungus, Rhytisma acerinum. The leaves are opposite, 

 and at an acute angle with the axis of the branch. A 

 glance at Fig. 9 will show how beautifully Maple leaves 

 are adapted to their conditions. The blades of the leaves 

 of the upper pair form an angle with the leaf-stalks, so 

 as to assume a horizontal position, or nearly so ; the leaf- 

 stalks of the second pair decussate with those of the 

 first, and are just so much longer as to bring up that 

 pair nearly, or quite, to a level with the first ; the 

 third pair decussate with the second, and are again 

 brought up nearly to the same level and immediately 

 to the outside of the first pair, so as to form a some- 

 what flat arch. In well-grown shoots there is often a 

 fourth pair on the outside of the second. If we look at 

 such a cluster of leaves directly from in front we shall 

 see that they generally appear somewhat to overlap ; 

 but it must be remembered that in temperate regions 

 the sun is never vertical. Moreover, while alternate 

 leaves are more convenient in such an arrangement as 

 that of the Beech, it is more suitable in such cases 

 as the Sycamores and Maples that the leaves should 

 be opposite, because if, other things remaining the 

 same, the leaves of the Sycamore were alternate, the 

 sixth leaf would require an inconvenient length of 

 petiole. 



