270 A TEXTBOOK OF THEORETICAL BOTANY 



the vertical growth of branches from this sub-hymenial layer. The branches 

 so formed divide repeatedly and penetrate between the paraphyses. The 

 end cells of these branches contain deeply staining protoplasm and usually 

 only two nuclei. These end cells curve over and form hooks, and in each 

 hook the two nuclei divide into four. This is followed by the septation of 

 the hook into a row of three cells. One nucleus enters the terminal cell, one 

 the basal, and two remain in the middle cell. This middle cell enlarges and 

 becomes the ascus, in which the two nuclei fuse (Fig. 261). Meiosis of the 

 fusion nucleus follows and by a further division eight nuclei are formed, 

 around each of which an ascospore is organized. 



These ascospores are oval in shape and brown in colour. They are ejected 

 forcibly from the apex of the ascus, and occasionally a tiny cloud of spores 

 may be observed around a fruiting body when large numbers of ascospores 

 are being expelled together. 



It will be noted that the two nuclei which fuse in the ascus have been 

 produced apogamously from vegetative hyphae and there is no evidence that 

 any sexual apparatus is developed. By analogy with other types it may well 

 be supposed that a hyphal fusion occurs somewhere prior to the production 

 of a fruiting body, but work along this line has not yet been done. 



Phacidiales 



The Phacidiales are Ascomycetes in which the apothecia are immersed 

 in the fungal tissue which is partly buried in the host tissue. They are 

 generally small in size and of a leathery or corky consistency. The group 

 is a small one but includes several genera which produce spots on the leaves 

 of higher plants. It should be noted, however, that many of the common 

 leaf-spot diseases are not caused by members of this order. Some of these 

 diseases are caused by members of the Sphaeriales in which the ascocarp is 

 flask-shaped, whereas in the Phacidiales it is more or less flat and widely 

 open. Others, and indeed the majority of leaf-spot Fungi, are placed among 

 the Fungi Imperfecti because they reproduce only asexually, by conidiospores. 



We shall consider one example of this order, Rhytisma acerinum. 



Rhytisma acerinum (The Black Spot Disease of Maples) 



This Fungus is extremely common throughout the country, attacking the 

 leaves of the common Field Maple and also the Sycamore ; in fact, few trees 

 and indeed few leaves escape. Superficially these leaves appear to have been 

 spotted with large drops of tar, and it seems that the lower leaves are usually 

 more frequently attacked and have larger spots than those at the top of the 

 trees. These black spots occur on the upper surface and average about 

 15 mm. in diameter (Fig. 262). They consist of a wrinkled stroma, roughly 

 circular in shape, which is formed mainly in the epidermal cells of the leaf, 

 though the hyphae spread down into the cells of the palisade layer as well. 



The mycelium arises from a germinated ascospore which forms a germ 



