520 ALTERATION OF FORM BY PARASITIC 1 'I'XUI. 



spore of the parasite. Parasitic species of Synchytrium cause a vesicular enlarge- 

 ment of single cells of the epidermis in the leaves of phanerogamic host-plants. 

 The not uncommon species Synchytrium Anemones and S. Taraxaei produce only 

 a slight overarching, and the enlargement of the cells is hardly more than four 

 times, often only twice the usual size. But, by the influence of Synchytrium 

 Myosotidis, hypertrophied epidermal cells rise up from the leaves of the Forget- 

 me-not (Myositis) in the form of comparatively large, club-shaped, bottle-like, or 

 egg-shaped bladders of golden or reddish yellow colour, and each contains the 

 parasite, or rather its spores. The parts of the leaf attacked by Synchytrium 

 Myosotidis are also much thickened, the palisade cells and the air-containing 

 lacunae of the spongy parenchyma (cf. vol. i. p. 279) disappear, and the tissue 

 consists entirely of large similarly-shaped cells which fit close to one another, 

 leaving no spaces between. In the gall caused by Synchytrium pilificum on 

 Potentilla Tormentilla the much-enlarged cells in which the parasite settles are 

 overgrown by the adjoining hypertrophied cells, some of which rise up in the form 

 of hairs, and the whole new structure resembles a hairy wart. 



A curious gall is produced by Exobasidium Rhododendri on a sharply-defined 

 portion of the foliage-leaves of the Alpine Rose (Rhododendron hirsutum and 

 ferrugineum). A spherical spongy body rises from a restricted portion of the 

 leaf, usually from the under side of the somewhat projecting midrib, sometimes 

 only as large as a pea, sometimes as big as a cherry, and occasionally even attaining 

 the dimensions of a small apple. It is yellow, but rosy-cheeked like an apple on 

 the side turned to the sunlight, and it reminds one of this fruit by its succulent 

 tissue and sweet taste. Indeed, these galls are sometimes called " Alpine Rose- 

 apples ". Their surface is covered with a bloom which is caused by the numerous 

 spores developed there and does not consist of wax like the bloom on an apple 

 rind. The neck joining the gall to the leaf is not more than 1-2 mm. across, and, 

 what is still more remarkable, except for this sharply-defined place of connection 

 the infected leaf is unaltered. 



Galls produced by the Gymnosporangia on the leaves of the Mountain Ash, 

 Pear-tree, Rock-medlar, and other Pomeae exhibit strange forms. One of them, 

 caused by Gymnosporangium conicum, on the foliage of the Rock-medlar (Aronia 

 rotundifolia), is represented in fig. 357 2 . It resembles a tubercle furnished with 

 horns projecting from the lower surface of the leaf. Microscopic examination 

 shows that the knob consists of the strangely metamorphosed spongy pareneh\ r ma 

 of the leaf. The intercellular spaces which normally contain air are quite filled 

 with the mycelial threads, and in the projecting portion of the tubercle, which 

 is very hard and almost cartilaginous, tubes are inserted which terminate blindly 

 below, where the spores of the parasite are developed, whilst above they are open 

 and fringed, thus allowing the spores to escape. These tubes look like horns to 

 the naked eye. Usually several galls occur together on the same leaf. They are 

 conspicuous at some distance on account of their colour. The chlorophyll is 

 destroyed wherever the mycelium of the parasite extends and a reddish-yellow 



