192 SIGNIFICANCE OF MALFORMATIONS IN ORGANOGRAPHY 



that these are operative when the cause of the malformations is either 

 an animal or a parasitic fungus. 



With regard to the changes brought about in the form of a host- 

 plant by an attack of parasitic fungi it is scarcely necessary to recall 

 here the remarkably varying degree in which they appear ; they are often 

 so slight that there is scarcely anything to be seen of them on the 

 outside. We should naturally expect a much stronger effect where the 

 parasite influences embryonal tissue, as happens especially at the vegeta- 

 tive points. The outgrowths which appear on many shoot-axes and 

 leaves must remain unnoticed here a , as well as all the numerous cases 

 of abortion. I shall only notice some examples in which the whole form 

 of an organ is affected by the parasite in such a way as to be designated 

 a malformation. We have in these cases to do with not merely the trans- 

 formation of an organ, but with the development of organs which might 

 otherwise remain latent, and also with the new formation of organs. 



Upon the branches of the silver fir, as well as upon other species 

 of fir, ' witches' brooms ' frequently appear, that is to say, negatively 

 geotropic shoots bearing needle-leaves which are annual in duration in 

 contradistinction to those of the normal shoots, and are also distinguished 

 from the normal leaves by structure and form. These abnormal shoots 

 are caused by the penetration into a bud of the mycelium of Aecidium 

 elatinum. They are always sterile, as are the shoots of species of Euphorbia 

 when they are attacked by Uromyces pisi, and take on in consequence 

 a habit strikingly different from the normal. Species of Exoascus may 

 also cause abnormalities in the branch-system of the cherry, birch, and 

 other trees, which have, too, been designated ' witches' brooms V 



If Peronospora violacea attacks the flowers of Knautia arvensis there 

 is often induced a transformation of the primordia of the stamens into 

 violet petals, that is to say, a ' doubling ' of the flower 3 ; in other cases 

 there may be only an arrest in the development of the staminal leaf or 

 the appearance of a petal-like wing in place of a pollen-sac. Giard found 

 that in Saponaria officinalis in like manner flowers which were attacked by 

 Ustilago antherarum had their stamens sometimes transformed into petals, 

 a transformation which, as we shall see, frequently happens on account of 

 the attack of animals. 



These are examples of a divergence in their development experienced 

 by the primordia of organs which would otherwise unfold in a different 



1 The mycological literature should be consulted for these ; see particularly De Bary, Comparative 

 Morphology and Biology of the Fungi, Mycetozoa and Bacteria, and the comprehensive account by 

 von Tubeuf in his ' Diseases of Plants induced by Cryptogamic Parasites.' 



2 See the many figures in Tubeuf's book cited in the preceding note. 



3 De Bary, Comparative Morphology and Biology of the Fungi, p. 368; Molliard, Cecidies 

 florales, in Annales des Scienc. Nat, ser. 8, i. 



