MORPHOLOGY 167 



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

 PHYTOPATHOLOGY. When the pathological structures are alone in question it may 

 he termed pathological morphology. Malformations which affect the external form 

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

 logical changes in the form and contents of the vegetable cells and 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 EUNST KUSTKU 

 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. 151) 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 carbohydrates in these tissues. A modified part of a plant in many cases 

 appears as if due to an arrested development. Dwarf forms and defective 

 differentiation 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. 



