THE TWO RUST DISEASES OF THE SPRUCE. 189 



in October, some time after its maturity, and the greater part of 

 the spores had already been shed. An examination showed that 

 in addition to the aecidia small yellow spermogonia are present. 

 The number of aecidia present on a leaf varies. 



Fig. 2 is from a photograph of a leaf bearing two pseudo- 

 peridia; several other aecidia were present, but the pseudo- 

 peridia surrounding them had fallen away. The pseudoperidium 

 consists of a single layer of thick-walled pitted cells with strongly 

 verrucose walls (see Fig. 3. in which the upper cells are shown 

 in surface view and the lower in optical section). The aecidio- 

 spores are subglobose or ellipsoid, usually with a somewhat 

 flattened side 20 - 24 x 20 - 23 /x in size. The wall is strongly 

 verrucose and pitted except at the flattened portion ; Fig. 4 

 shows a spore in surface view, and Fig. 5 one in optical section. 

 The peculiar flattening of the aecidiospore depends on its 

 method of development. Numerous rows of cells are produced 

 in the young aecidium, and in each row only every alternate 

 cell gives rise to a spore. The intermediate cells which do not 

 produce spores remain thin-walled and at maturity become 

 gelatinous and almost disappear. The flattened part of the wall 

 of the aecidiospore is the portion which was originally in contact 

 with the intermediate cell below it, and, in some cases, the 

 shrivelled remains of this cell are still attached and may be seen 

 as a black line when the aecidiospore is seen in optical section 



(Fig. 5)- 



In districts where the spruce does not occur it is probable 

 that the fungus may exist through the winter in the form of 

 hibernating uredospores, which, in the following spring, can 

 infect the rhododendron ; it appears therefore that the presence 

 of the spruce is not essential to the continued existence of the 

 fungus. The aecidial stage on the spruce, however, can only 

 exist where the rhododendron is present, as the infection of 

 the needles is only brought about by the sporidia. Since this is 

 the case, the removal of the rhododendron will completely check 

 the disease. 



It appears, therefore, that the spread of the disease on the 

 spruce will be limited in this country since Rhododendron 

 hirsutum and R. ferrugineum^ although frequent in parks and 

 gardens, are not usually grown in plantations. The fungus 

 has been found on R. dahurtcum, but does not attack R. ponticuniy 

 R. catatvbierse, nor their hybrids, which are so commonly grown. 



