770 TRANSACTIONS OF SECTION K. 



chromosomes arise by a condensation of the chromatin. At about the stage of 

 the segmentation of the nuclear thread contraction figures can be observed. 

 Some of the nucleolar chromatin appears to be taken up by the chromosomes, 

 but the nucleoli themselves are extruded into the cytoplasm when division 

 takes place. 



The spireme often exhibits a folding at the periphery of the nucleus as 

 described by Farmer and Moore in the nuclei of higher plants, but this did not 

 resolve itself into bent and twisted segments, although appearances were observed 

 which might have been interpreted in this way. 



Reduction is brought about simply by the distribution of the chromosoires 

 into groups of four each to the two daughter nuclei at the first division of the 

 basidial nucleus. This apparently corresponds, therefore, to the brachymeiotic 

 phase in the ascus as described by Miss Fraser, and seems to indicate that a 

 sexual fusion necessitating a meiotic phase is absent. The second division in 

 the basidium appears to be normal, in that four chromosomes are produced, 

 which divide into two groups of four each to form the nuclei which will 

 ultimately pass into the spores. The four chromosomes in the daughter nuclei 

 were in most cases very clearly seen at the poles of the spindle. In the re- 

 constitution of the daughter nuclei the chromosomes first of all fused together, 

 and out of this fused mass the nuclear network and the nucleolus were 

 differentiated. 



Clear indications of the four chromosomes were also seen in the nuclear 

 division in the spores. 



5. Some Observations on the Silver-Leaf Disease of Fruit Trees. 

 By F. T. Brooks, M.A. 



This disease attacks plum trees more frequently than any other kind of fruit 

 tree. Apple and cherry trees are occasionally affected, and I have recently found 

 a few red-currant and gooseberry bushes attacked by this disease. 



As the name of the disease implies, the foliage of affected plants presents a 

 silvery appearance in contrast with the dark green colour of healthy leaves. 

 At the commencement of attack a single branch usually becomes affected, but 

 sooner or later the entire foliage assumes a silvery appearance. After an attack 

 during one or more seasons some of the branches begin to die, and ultimately 

 the whole tree succumbs. This disease seems to be rapidly increasing in frequency, 

 and in some fruit plantations of Cambridgeshire nearly 10 per cent, of the plum 

 trees are affected. 



Percival ' appears to have been the first to call serious attention to this 

 disease. In 1902 he showed that the silvery appearance of the leaves was due, 

 not to any change in the constitution of the chlorophyll, but to the formation 

 of air-cavities in certain of the walls of the epidermal cells. He found that 

 portions of roots of affected trees gave rise to the sporophores of Stcreum 

 purpureum when kept in a moist atmosphere. Upon inoculating branches of 

 healthy trees with portions of these sporophores he found that the foliage 

 of the inoculated branch subsequently became silvered. He concluded, there- 

 fore, that Stereum purpureum was the cause of the disease, and considered that 

 this fungus secreted an enzyme which, upon being carried up the branch, 

 produced the silvering of the foliage. More recently Spencer Pickering" has 

 made a number of inoculation experiments with sporophores of this fungus, and 

 in the large majority of cases silvering of the foliage of inoculated trees 

 subsequently resulted. On the other hand, some observers doubt whether 

 Slereum purpureum is the cause of the disease. 



For the last two seasons I have been making observations and experiments 

 on this disease among the fruit plantations of Cambridgeshire. The work is 

 not yet complete, but the results which have so far come to hand are as 

 follows : — ■ 



Large numbers of plum trees have been seen on the dead branches of which 

 the sporophores of Stereum purpureum were present in abundance; in these 



1 Percival, J num. Linn, Sor., 1902, vol. xxxv. 



* Nnenrer Pickering. Report of the Woburn Experimental Fruit Farm. 190Cv 



