1 88 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



development of one species to which he assigned the present 

 name. 



This species is found frequently in the Alps wherever the 

 Alpine Rose {J^hododendron hirsutum and R . ferrughieuni) occurs. 

 The yellow clusters of uredospores are developed in September 

 on the lower surface of the leaves and also on the bark of the 

 shoots of the previous year. The uredospores are oval in form 

 and are produced in chains ; they may further propagate the 

 disease on the rhododendron. Slightly later the development 

 of the teleutospores commences, and in the following spring 

 the sori appear as small dark red cushions on the lower surface 

 of the leaf. 



The teleutospores are formed close together in groups 

 covered by the epidermis, and each spore consists of a series 

 of superimposed cells. A section of a mature sorus is shown in 

 Plate XX. Fig. i. Immediately before germination the epidermis 

 is ruptured and the terminal cell of the teleutospore, the only 

 one capable of germination, gives rise to a four-celled pro- 

 mycelium, each cell of which produces a sporidium. The sporidia 

 are set free in June, and if they alight on the young leaves 

 of the spruce may cause infection. A well-developed mycelium 

 is produced in the leaf, and the infected area becomes yellow in 

 colour. In some cases almost the whole of the leaf is infected, 

 but generally the fungus is confined to certain zones and the 

 remaining portions retain their normal green colour. As a 

 result of infection small yellow spermogonia are first produced, 

 and these are soon followed by the aecidia. Each aecidial 

 sorus is surrounded by a long white pseudoperidium which, 

 in the early stages, completely encloses the spores. The aecidio- 

 spores are arranged in chains and are produced from the base 

 of the aecidium ; they are orange-yellow in colour. At maturity 

 the pseudoperidium breaks down at its apex and allows the 

 aecidiospores to escape as a powdery orange-yellow mass. 

 Before dehiscence the pseudoperidium has the form of a cylinder 

 terminated by a rounded cone and possesses a yellow tint due to 

 the colour of the enclosed spores. After the spores are shed 

 the form is that of an open cylinder and the pseudoperidium is 

 perfectly white. The aecidiospores are distributed by the wind, 

 and if they alight on the leaves of the rhododendron are capable 

 of producing infection. 



The material of the aecidial stage of the fungus was obtained 



