54 



THE DICTIONARY OF GARDENING, 



Fear continued. 



called peridia, and contain minute, nearly globular, simple 

 Bpores, which escape, when mature, through fine slits in 

 the sides of the peridia. This Fungus cannot be called 

 common in England, and it has not been recorded from 

 Scotland ; but in many Continental localities, it is so 

 abundant as to do much injury to the trees. 



FIG. 56. PEAR LEAVES ATTACKED BY ROESTET.IA CANCELLATA 

 (believed to be a stage in the development of Gymnosporan- 

 gium Sabince)a, Upper Surface, and 6, Lower Surface, of 

 Leaf ; sp, Spot on Upper Surface, surrounded with dark 

 border; p, Spot on Lower Surface, bearing Peridia, as de- 

 scribed in the text. 



Some years ago, the Danish botanist, Oersted, made 

 investigations into the development of this Fungus, which 

 led him to the result, now very generally accepted by 

 botanists, that it is only a stage in the life-history of a 

 Fungus which was regarded in former times as entirely 

 disconnected with the Roestelia, and which grows on the 

 branches of certain species of Juniper, notably of the 

 Savin (J. Sabina). This latter Fungus, known as Podi- 

 soma fuscum, Cda., or Gymnosporangium fuscum, DC., 

 induces a thickened state of the branches of its host. 

 From this project numerous cylindrical or bluntly conical 



FIG. 57. JUNIPER TWIG ATTACKED BY GYMNOSPORANGIUM 

 FUSCUM-C, Outgrowths, in which Spores are formed. 



outgrowths, often fin. to l$in. long (see Fig. 57, c). 

 These outgrowths are at first yellow, but become brown ; 

 and, when moistened, they assume the consistence of 

 jelly. The microscope shows that they are made up of 

 colourless, one-celled filaments, imbedded in the jelly, 

 and running from within outwards, each of which bears at 

 its tip a small spore of an elliptical or biconical form, 

 with a division wall in the middle, so that it is made 

 up of two cells, base to base, as in Puccinia. The 

 spores lie on the surface of the body, and, when it dries, 



continued. 



they arc blown about by the wind. When one germi- 

 1 nates, it forms one or two filaments, made up of a row 

 I of cells ; and from each of the cells, from the tip of this 

 I for a little way down, a branch grows, and forms on its 

 tip a minute sporidium. These are believed, when they 

 fall on Pear leaves, to give rise to the Roestelia cancel- 

 lata. The spores of this, in turn, are now believed to 

 be the cause of the Gymnosporangium, when they fall 

 on the proper hosts. This phenomenon of a complex 

 cycle, believed to exist, peculiarly well marked, in many 

 species of Puccinia, is further referred to under that 

 heading. The diseased Pear leaves should be removed 

 and destroyed as soon as they show well-marked signs 

 of the Fungus, and before the spores are scattered 

 from them. The species of Juniperus that nourish Gym- 

 nosporangium should not bo permitted to grow near 

 Pear-trees, especially if the Fungus shows itself on the 

 Juniper bushes. 



In August of 1885, a communication was made 

 to the American Association for the Advancement of 

 Science, by Mr. J. C. Arthur, regarding the cause of the 

 disease known as Pear Blight. This disease attacks 

 Apple as well as Pear trees, and causes the death of 

 the diseased parts. A yellowish, viscid substance is 

 formed, apparently at the expense of the starch in the 

 branch or other part attacked. Prof. Burrill found that 

 the diseased tissues invariably contained a microscopic 

 Fungus, named by him Micrococcus amylovorus, belonging 

 to the group of Schizomycetes, so prominent of late 

 years as the causes of disease in man and in domestic 

 animals. Mr. Arthur has made numerous experiments, 

 which have convinced him that the Micrococcus is the 

 direct cause of Pear BligLt. Cure of the disease, after 

 it is once established in a branch, seems impossible; 

 but prevention of its spread should be aimed at, by the 

 removal and burning of all parts that show any signs of 

 the disease. Infection of healthy plants is easily accom- 

 plished if cracks or wounds on their surface are brought 

 into contact with any of the diseased tissues. 



"Cracking" in Pears, and in Apples, may be due to 

 more than one cause ; but one of the worst forms is the 

 work of a Fungus, which, on Pears, has been called 

 Fusicladium pyrinum, Fckl., but is now generally re- 

 ferred, as a variety, to F. dendriticum, Fckl., now more 

 often called Cladosporium dendriticum, Wallr., which, 

 in its typical state, grows on Apples. This Fungus grows 

 on the leaves and young twigs, and also on all parts of 

 the flowers, often preventing the fruits from setting; 

 and if they do set, they are crippled and injured in 

 growth, and are rendered of little value by the Cracking 

 of the skin of the fruit as it ripens. On the leaves, the 

 Fungus gives rise to black spots, from iin. to in. across, 

 branching from the centre like a minute tree, whence 

 the name dendriticum, or tree-like. On the fruit, it gives 

 rise to similar spots, which very soon become irregularly 

 rounded, with a narrow, white margin, surrounding a 

 depressed black spot. Around the white margin there 

 is a dark border, due to the spreading mycelium of the 

 Fungus. On examining the spots with a low magnifying 

 power, it is evident that the margin is the edge of the 

 epidermis or skin of the fruit, and that the dark, de- 

 pressed spot is occupied with a mass of minute bodies. 

 On using a lens of higher power, it is seen that these 

 bodies are the ends of club-shaped or narrowly-ovate, 

 brown spores, or conidia, which are fixed by the narrow 

 end to the tip and sides of erect, short stalks. The 

 conidia are simple, or are occasionally two-celled. The 

 stalks are produced on the surface of a mass or stroma 

 of closely-packed small cells, formed by division of the 

 threads of the mycelium by cross walls. The mycelium 

 does not penetrate far into the fruit. It remains almost' 

 wholly in the cells of the epidermis, and between them 

 and the next layer ; though a few branch-threads are 



