SECT. I MORPHOLOGY 171 



its further development as a stem. Leaves, even when somewhat 

 advanced in growth, may under changed conditions vary their nsual 

 character, particularly within the limits of their possible meta- 

 morphosis ; for example, the staminal and carpellary leaves of a 

 flower may thus become transformed into additional perianth leaves. 

 The later the rudiments of an organ are acted upon by a disturbing 

 influence, so much the less far-reaching are the modifications which 

 are produced ; thus intermediate forms betv/een two organs may be 

 produced which correspond more or less closely to one or the other of 

 them. Neither the abnormal interchangeability, at times manifested 

 between morphologically different members, nor the development of 

 intermediate forms between them, can be considered as proof of their 

 phylogenetic connection. Malformations are, accordingly, to be 



ACCEPTED AS EVIDENCE IN MORPHOLOGICAL QUESTIONS ONLY IN THE 



RAREST CASES. When, as is only exceptionally the case, they represent 

 reversions, this value may be attached to them. 



The study of the abnormal formations and functions of the plant is called 

 Phytopathology. When the pathological structures are alone in question it may- 

 be termed pathological morphology. Malformations which affect the external form 

 are still treated by themselves as Vegetable Teratology (Fhytoteratology). Patho- 

 logical changes in the form and contents of the vegetable cells aud tissues form the 

 subject matter of pathological plant anatomy. 



The study of the aetiology of abnormal vegetable tissues shows that various 

 external influences may give rise to them ; the most important causes are the in- 

 fluence of wounding and of the attacks of parasites. According to Ernst Kuster 

 abnormal cells may be characterised by their contents, the thickening of their 

 walls, or by their unusual size (hypertrophy) ; in the last case they may also show 

 divisions which would not normally occur. The abnormal tissue may resemble 

 that from which it originates or deviate more or less widely from it. The tissues 

 formed in the healing of wounds (cf. p. 162) and those of galls may be mentioned as 

 examples. The tissues of galls are characterised more by the increase in number 

 than by the size of their cells, which may undergo the most various modifications. 

 There is often a considerable accumulation of albuminous substances and carbo- 

 hydrates in these tissues. A modified part of a plant in many cases appears as 

 if due to an arrested development. Dwarf forms and defective diflerentiation of 

 tissues, with its accompanying impairment of function, are examples of this. In 

 all cases in which the cells of a tissue no longer present their normal properties 

 they are liable to undergo sooner or later regressive changes leading to degeneration 

 and necrosis. 



