380 GENERAL PLANT PATHOLOGY 



differentation in the callus. The callus of poplar cuttings is favorable 

 for a study of its formation. The outer cells of the wedge of healing 

 are long and pouch-like, and their outer walls give the cork reaction, 

 since they take up Sudan III with avidity, and at the same time are 

 colored with hydrochloric acid and phloroglucin. Sooner or later, a 

 cork cambium is produced in the outer cell layers of most callous for- 

 mations. Massart, who first studied the nuclear phenomena in callous 

 tissue, rarely found that the cells contained more than one nucleus. 

 He found that direct nuclear division took place after wounding in 

 Cucurbita, Ricinus and Tradescantia, while Nathansohn found mitosis 

 in the callus of the divided roots of Vicia faba and both mitosis and 

 amitosis in that of poplar cuttings. 



Conditions of Callous Formation 



The behavior of cuttings from different plants varies within rather 

 wide limits. Some cuttings develop callus quickly, others slowly, and 

 the quality of the callous tissues differs as greatly. The poplar develops 

 a large amount of callus, while cuttings of elm, willow and oak form 

 only a low callus ring. Organs rich in foodstuffs form callus more 

 quickly than those poor in food materials. For example, the cotyle- 

 dons of Phaseolus and Vicia, rich in proteins and starch, develop callus 

 to an extraordinary degree. Moisture is an important factor in the 

 formation of callus, for it is formed in water, but better in moist air, 

 and not at all in dry air. Cuttings of poplar with both cut ends in 

 moist air develop callus at both extremities, but usually there is a 

 polarity shown. Cut-off petioles of the poplar form a more prolific 

 callus at the basal end of the petiole than at the end nearer the leaf 

 blade With stem cuttings, the callus is best developed at the basal 

 end in preference to the apical. Pieces of dandelion roots, 3 cm. long, 

 kept in a moist place, show most abundant callus on the upper stem 

 ends and not at all, or only slowly at the apex, but in alfalfa a power- 

 ful tuber-like callus is produced at the root end and feebly at the sprout 

 end. So that having varied the external conditions of their formation, 

 it becomes evident that internal conditions are active and these prob- 

 ably depend upon inequalities in the nutritive condition of the cut 

 parts and also on the direction of established sap flow. 



Loosely connected with pathologic anatomy ^re the regenerative 



