INTRODUCTION 13 



known are those induced by Exoascus turgidus on the Birch, 

 and by Pevidermium elatinum on Silver First (Plate XVII.). 



Galls on Algae, Mosses, and Lichens 



A. W. Bennett has described a gall occurring on the alga 

 Vaucheria Dillwyni Agardh., probably caused by the rotifer 

 Notommata WernecU Ehrenb. Similar galls on various 

 species of Vaucheria have been recorded by Continental 

 observers. This gall is of very variable size and aspect, 

 often assuming the form of an elongated capsule bearing 

 numerous prolongations or tubercles on its surface ; the 

 causer may be discerned, with the aid of a lens, within 

 the gall, appearing as a little black point. A copepod 

 {Harpacticus chelifer O. F. Muller, according to Barton) 

 galls Rhodymenia palmata, causing numerous papules on the 

 thallus. A similar agent deforms the thallus of Desmarestia 

 aculeata Lamouroux. 



Connold depicted ("Plant Galls," Fig. 278) stems of 

 Halidrys siliqua Lyn. with pronounced globular swellings. 

 They were picked up on the beach at St. Leonards in 1902, 

 between November and April, after heavy storms. " Many 

 efforts were made to determine the cause, but without 

 success." 



Eelworms cause galls on algae and on mosses ; for notes 

 concerning these, see Chapter VIII. There is a reference 

 in Chapter VII. to galls on lichens supposed to have been 

 caused by mites. 



The majority of the agents causing galls on British 

 Phanerogams and vascular Cryptogams are insects included 

 in the orders Hymmoptera, Cokopiera, Lepidoptem, Diptera, 

 and HomopUra. These and the galls caused by Mites 

 {Acari), Eelworms {Nematoda, family AnguilluUdae), and 

 Fungi are discussed in the following chapters. For the 

 most part, familiar galls only are described, the text being 

 chiefly descriptive of the plates. 



