524 ALTERATION OF FORM BY PARASITIC FUNGI. 



m,vi (see fig. 358 =) are seven times as long as broad and linear in shape. They 

 stand erect, and are of a much paler colour than the healthy leaves. The Wood 

 Anemone (Anemone nemorosa) affords another example (see fig. 259, p. 229). It 

 spreads by creeping stems under the surface of the ground, and forms small 

 colonies in light thickets and in meadows. The plants consist partly of flowering 

 lateral shoots, and partly of foliage-leaves, which emerge above the ground from 

 the creeping underground stem. In normal leaves the erect petioles are all the 

 same length, and the leaflets are extended at about the same level. But when the 

 ^oidium stage of Puccinia fusca has settled on them this becomes altered. The 

 Hades of the infected leaves tower over their healthy neighbours in consequence of 

 the elongation of their petioles, whilst their leaflets are smaller and less divided. 

 The length of the petiole in normal leaves is some 12-13 cm., in hypertrophied 

 leaves 15-18 cm.; but the size of the altered segments, compared with those of 

 normal leaves, is as 5 : 7. Similar changes are observed in leaves of SoldaneUa 

 alpi/na when attacked by Puccinia Soldanellce. The petioles of the infected 

 leaves are 2-4 times as long as the normal ones, the blade is smaller and 

 hollowed like a spoon instead of being flat, and the colour is an ochreous yellow 

 instead of a dark green. The same alterations in the length of the petiole, 

 and, in the size and colouring of the- leaf -lamina, are produced in the leaves of 

 Alchemilla vulgaris by Uromyces Alchemilloe and in those of Pkytewma orbi- 

 culare by Uromyces Phyteu/matum. To this class belongs also the so-called 

 " curl " disease of Peach and Almond trees, produced by Exoascus deformans, and 

 rendered conspicuous by the considerable enlargement, undulation, and bladder- 

 •like expansion of the infected leaf-surface, which acquires generally a very brilliant 

 coloration. 



Floral-leaves are comparatively seldom metamorphosed by Fungal parasites. 

 In the Alder {Alnus glutinosa and incana) the bracts of the pistillate flowers are 

 changed by Exoascus Alni-incance (=:E. amentorum) into elongated purple-red 

 spatulate lobes much twisted and bent (see fig. 358 1); Peronospora violacea some- 

 times causes the stamens to change into petal-like structures in the flowers of Knautia 

 arvensis, so that they then seem to be " double "; Ustilago Maydis causes a growth 

 of tissue in the pistillate flowers of the Maize, the result being that instead of grains 

 irregular cushion-like structures 7 cm. in diameter are produced. Taphrina aurea, 

 which settles on the pistillate flowers of Poplar (Populus alba and tremula) causes 

 the ovaries to form golden-yellow capsules more than twice the usual size. The 

 galls produced by Exoascus Pruni on the ovaries of wild Plum, Bullace, Sloe, and 

 Bird Cherry (Prunus domestica, insititia, spinosa, Padus) belong also to this class. 

 The tissue of the ovary increases in size, but not in the same way as in fruit forma- 

 tion. The resulting body is flattened on two sides, brittle and yellow; the seed 

 inside is abortive, and a hollow space is left in its stead. The gall produced from 

 the ovary of Prunus domestica has the form of a rather curved pocket, which 

 looks as if it had been powdered outside with flour at the time the spores ripen. 

 These hypertrophies, which are popularly termed "pocket-plums", "bladder-plums", 



