22S 



HARDWICKE'S SCIENCE-GOSSIP. 



air is circulated in the cool cellar, the air is cooled, and 

 deposits water proportionally, as it comes in contact 

 with the cool walls and roof and floor. We usually 

 blunder all round in this country, probably owing to 

 our exceptional experience of hot weather. In Spain, 

 Italy, and other countries, where hot weather is 

 better understood, windows and doors, and even 

 window-shutters are scrupulously closed during the 

 daytime in summer, are only opened in the night or 

 early morning. When we have a " scorcher," we 

 throw open doors and windows, in order that the 

 scorching may proceed both indoors and outside, and 

 that escape may be rendered quite impossible. 



Fries, but the state in which it is usually seen is that 

 originally called Melasmia acerina, Lev. The spots 

 begin to appear in June, and towards the end of July 

 become very conspicuous. When a tree is attacked, 

 usually nearly every leaf is more or less spotted, and 

 the number of spots on each leaf may reach thirty or 

 even more (Fig. 148). They begin with a yellowish- 

 discoloration, of a roundish form, about a quarter of 

 an inch in diameter. The yellow spots are caused by 

 the influence of the mycelium of the fungus on the 

 chlorophyll of the leaf; this mycelium grows centri- 

 fugally, just beneath the epidermis, probably from a 

 stomate, and destroys the colour of the chlorophyll as 



<:~ 



Fig. 148. — Leaf of Sycamore, with black ungus spots. 



FUNGUS SPOTS ON SYCAMORE LEAVES. 



IN answer to the inquiry that was made in one of the 

 numbers of Science-Gossip as to the cause of 

 the spots on sycamore leaves, several have been 

 already suggested. The most frequent cause of spots 

 on the leaves of that tree is the fungus which I 

 purpose to describe in this article. With the aid of 

 the figures I have drawn from nature, it will be easy 

 to decide whether the spots seen in any case are due 

 to this or any other source. The same fungus occurs 

 on maple leaves, and similar spots, due to an allied 

 fungus, are not uncommon on willow leaves. 



The fungus to which I refer is Rhytisma acerinum. 



Fig. 149. — Melasmia acerina (section through a spermagone}. 

 X 70. 



it advances. Ic is confined to the yellow spot ; in 

 the still green parts of the leaf no mycelium can be 

 detected. 



So far the epidermis of the leaf has remained un- 

 affected. But now certain parts of the interwoven 

 mycelium or hyphasma begin to arrange themselves 



