Co lour- Changes in Leaves 



397 



spring, (a) Leaf with fungus spots, but without autumnal 

 colour changes, (b) Leaf with fungus markings, also autumnal 

 colouring caused by excess of oxygen. The fungus causes 

 early death and fall of the leaf. It also occurs on maple, and 

 various species of Acer in other countries. 



Sycamore. 



Sycamore leaves with the characteristic yellow tint assumed 

 in old age, but retaining here and there bright green spots and 

 patches on the upper surface, with greyish patches on the 

 corresponding lower surface. These markings are induced by 

 the fungus Botrytis deprczdens. 



Black Currant. 

 Autumnal colour-changes in leaves of the black currant. 

 On the lower surface may be seen the fungus known as 

 Cronartium ribicola. It is the winter state of the Weymouth- 

 pine fungus (Peridermium stvobi) apparently it causes brighter 

 red tints than are usually observable in non-infected leaves. 



Elm. 



The leaves turn yellow and become brown after they have 

 fallen. The black spots so frequently seen on the yellow 

 leaves are the undeveloped form of the fungus Dothidea ulmi. 



Group IV.— CHANGES INDUCED BY ACCIDENTAL 

 CAUSES. 

 Bramble. 



Bramble leaves with spots superficially resembling those 

 induced by micro-fungi. The markings here depicted were 

 caused by sparks falling upon the leaves from a fire on Wey- 

 down, Haslemere, in April, 1901. Lens-like thickenings in 

 the glass of greenhouses will sometimes focus the sun's rays 

 upon leaves, and cause yellow or brown spots. The action is 

 that of a burning-glass, and if very intense the spots may 

 become black. 



