224 PHYTOHORMONES 



on sections of Populus twigs by separating and weighing 

 it. The amount of callus formed by auxin decreases rapidly 

 with distance from the point of application, approaching 

 zero at about 3 cm. below it. If applied simultaneously at 

 two points on the twig, the callus produced was equal to 

 the sum of the amounts which would be produced by single 

 applications at the same points; the method is thus satis- 

 factory for quantitative work. Auxin a was somewhat less 

 active in callus formation than indole-acetic acid. 



As to the histology of the thickening, it involves both 

 cell enlargement and division, the enlargement, which is 

 partly radial and partly isodiametric, being mostly confined 

 to the cortical parenchyma and the pith cells (u). Laibach 

 and Fischnich (1935) found that the proportion of divisions 

 to enlargement was always about the same in Vicia Faba, 

 and therefore use the total swelling as measure of the cell 

 division; ^ they correspondingly speak of the swellings as 

 ''callus." However, the term callus has usually been applied 

 to the formation of undifferentiated and random-oriented 

 cells at wound surfaces, whereas these swellings show at 

 least in part the character of the stem; further, since en- 

 largement of parenchyma accounts for a large part of the 

 increase in volume, and in some cases (as claimed by Czaja, 

 19356 ^) for all of it, it seems better simply to use the term 

 ''swellings." Within the swellings secondary lignification 

 of the cell walls may often be observed, so that it seems 

 possible that auxin controls or stimulates such secondary 

 wall formation. 



Czaja (1935) has put forward the view that the effect of 

 auxin depends largely on the direction of its application. If 

 applied directly to the transversal cut surface of the Helian- 

 thus hypocotyl he finds increase in elongation to be the prin- 

 cipal effect, but if applied externally, to the epidermis, the 

 principal effect is swelling. His published measurements, 



1 The controversy between Czaja and Laibach as to whether the auxin swellings 

 are due to cell division or enlargement is easily explained: when auxin is applied, 

 enlargement of the parenchyma cells takes place first, and cell divisions follow 

 after a few daya (Went, 1936). 



