ii6 BRITISH GALLS 



the only recorded instance of actinomycosis amongst plants. 

 The masses of tubercles vary in size from that of a pea to a 

 small walnut, but are usually never so large nor so abundant 

 as those of the Alder. A single tubercle is a root-like growth, 

 branching di- or trichotomously after attaining about 2 mm. 

 in length, each tip sending out a thread-like structure, some- 

 times 3 cm. long, much resembling an ordinary rootlet, 

 but tapering towards the tip. It is at first greyish-pink, 

 then flesh colour, becoming dark brown when exposed to 

 the air ; sometimes the tubercles are quite black when very 

 old. Mr. "William Herridge observed these tubercle masses 

 in abundance on the roots of Bog Myrtle in the clay-pits at 

 Newton Abbot in 1910, and they are probably of frequent 

 occurrence on this plant 



Fungi that cause reduction in size of the afflicted plants, 

 or abortion of certain organs, are termed AtrophyUs. With 

 such the cecidologist is not concerned, except in a few 

 instances in which the dwarfing is accompanied with 

 minute gall-like excrescences on various parts of the plant, 

 as may be seen in the case of Dandelion plants attacked by 

 Synchytrium taraxaci.. Under the influence of the parasite 

 the dwarfed plants produce minute pustular galls on the 

 leaves and involucres. 



Fungi causing enlargement of the entire plant, or hyper- 

 trophy of certain organs only, are termed Hypertrepkytes. 

 They may be either autoecious, going through the life-cycle 

 on a single host plant, or heteroecious, growing on different 

 host plants during the various periods of the life-cycle. 



Amongst the most remarkable of hypertrophies caused by 

 fungi on leaves are the cherry-like structures which appear 

 in July on the under surface of the leaves of Rhododendron 

 fevvaginmrn, Jmsutum, and Wilsonii, induced by Exobasidium 

 rhodedendri. The gall is a buljate excrescence, sometimes 

 attaining the size of a Walnut, pale at first, becoming reddish 

 and shining. It is known as the " Alpine Rose Apple " ; it 

 superficially resembles the " Cherry gall " of the Oak leaf, 

 but is sweet. This large structure is connected with the 



