374 GENERAL PLANT PATHOLOGY 



while in hy})crtiophy they are not. A number of well-defined groups of 

 vegetative hyperplasias may be distinguished by their etiology. Chemic 

 stimulation may be the cause of some, injury the cause of others. The 

 normal currents of foodstuffs may be clogged, the food may be irregu- 

 larly distributed and these interferences with normal processes may 

 result in proliferations and other abnormalities. Special stimuli may 

 also bring about abnormal supplies of food with consequent hyperplas- 

 tic tissue formation. The study of the abnormality to determine its 

 kind must be based on histologic analysis. If in our histologic examina- 

 tion, we discover that the abnormal tissues resemble the corresponding 

 normal plant parts, we are dealing with homooplasia; if they differ from 

 the normal, that is are composed of cells different from the correspond- 

 ing normal ones, then we have a case of heteroplasia. 



Heteroplastic excrescences are of great interest histologically. The 

 difference between normal and abnormal states is sometimes greatly 

 diverse. This difference may be one of size, of tissue differentiation, of 

 constitution, and it is important in our pathologic study to determine 

 the nature of the differences between normal and abnormal conditions. 

 Thus, when we find a less differentiated tissue produced by abnormal 

 cell division without regard to the increase in the numbers of cells, we can 

 speak of the degeneration of tissue formation combined with an increase 

 of volume. This is known as cataplasy, and the products of the cata- 

 plastic processes as cataplasms and the kind of hyperplasia illustrated 

 in these abnormal changes as cataplastic hyperplasia. When, on the 

 other hand, we find new histologic characteristics and functional activi- 

 ties associated with hyperplasia, we speak of prosoplasy, of prosoplasms, 

 and of prosoplastic hyperplasia. 



Homooplasia. — This term may be defined as abnormal tissue forma- 

 tion produced by an increase of the normal elements; it has a limited 

 use to abnormalities, not to increase in size of normal organs by a mere 

 increase in the number of cells. We would not use the word homo- 

 oplasia for the unusually large leaves which of normal form and texture 

 appear on the shoots which arise from tree stumps and which have been 

 studied by the writer in a number of our American forest trees, such as 

 the tulip tree, Liriodendron tulipijera. Homooplasia is opposed to the 

 phenomena of giant growth here mentioned. 



Localized tissue excrescences composed of the same histologic ele- 

 ments and of homooplastic character are not common. Occasionally 



