GROWTH 



311 



stances which stimulate division in the neighboring cells. If a 

 freshly cut surface of plant tissue is immediately washed with 

 water, then there seldom occurs any division or growth in the 

 cells adjoining the wound. But if the wound is smeared with a 

 paste consisting of minced tissues of the same organism, then 

 division increases considerably (Fig. 123). The substances thus 

 stimulating division are called by Haberlandt "wound hormones." 



Fig. 123. — Multiplication of cells on the surface of a wound of kohlrabi. Wound 



not washed (center) ; washed with water (right) smeared with a mass of minced 



kohlrabi (left) {redrawn after Haberlandt). 



They probably are specific, since material from the tissues of 

 another plant produces no effect. Another instance of a plant 

 hormone is the "growth-promoting substance" recently detected 

 by Wendt (see Art. 83), which seems to be an indispensable con- 

 dition for the elongation of plant cells. 



The hormones are considered as playing an important role in 

 the regulatory movements of plant organs. They may be like- 

 wise associated with the reproduction of plants. In the following 

 chapter examples of their effect will be given. 



