134 PLANT PHYSIOLOGY 



not reach their full size or number (hypoplasy) , or in which 

 individual cells or whole tissues are enlarged above the 

 normal size (hypertrophy), or in which the cells are ab- 

 normally increased in number (hyperplasy). In some 

 cases cells destined to produce one kind of tissue are 

 changed into other kinds by the pathological conditions. 

 Furthermore, the internal structures of the cell may be 

 modified. The chloroplasts may be increased in number 

 and size or diminished or apparently wholly suppressed. 

 The nucleus may be enlarged and changed in shape or 

 caused to divide abnormally so that multinucleate cells 

 result. The contents of the cells are often modified; 

 acids may be increased or diminished; the tannin content 

 may increase remarkably in some cases as also that of 

 various coloring matters or of various enzymes. 



196. These changes are in some cases the results of 

 causes not as yet recognizable. Such troubles are spoken 

 of as " Physiological Diseases, " this being simply a name 

 to cloak our ignorance of the true cause of the trouble. 

 In many cases, however, the changes occur as a result of 

 the action of parasitic organisms, either plant or animal 

 in nature. In the case of many inj uries caused by animals 

 (e.g. biting insects) the injury is chiefly mechanical and is 

 a subject for study from the standpoint of pathology in 

 just the same way as the study of wounds caused by other 

 agencies. But the punctures of some insects (e.g. plant 

 lice, aphids) are followed by marked physiological dis- 

 turbances in the cells immediately or even remotely ad- 

 jacent to the punctures, leading to the type of disease 

 called stigmonoSe (or puncture disease) . The enormously 

 varied structures found in insect galls as a result of the 

 presence or punctures of various gall-producing insects, 

 if properly understood, would doubtless throw a flood of 

 light upon the subject of pathology and even physiology. 



