Physiological and Anatomical Effects 123 



successive leaves are developed subsequent to auxin application to 

 peppermint plants. Leaves which are near maturity at the time of 

 treatment with 2,4-D will mature normally as seen in the right leaf 

 in the figure. Leaves which are somewhat less mature at time of treat- 

 ment characteristically show distortion of the lower half of the leaf, 

 where the mesophyll islets fail to develop normally, so that cladifica- 

 tion or expansion fails over the lower part of the leaf. Still younger 



Fig. 55. Distortions of leaves of peppermint which develop after application of 

 2,4-D (1,000 mg./l.). Leaf at left is normal, and successively to the right leaves show 

 inhibition of cladification, excessive enlargement of veins, shortening of leaves, and 

 development of marginal ruffles. The leaf at far right is normal, developed entirely 

 after the auxin effect was dispersed. 



leaves at the time of treatment show the failure of cladification over 

 the entire leaf, and the resultant leaf is a narrow structure with 

 nearly parallel veins. The veins are abnormally large, principally be- 

 cause of the formation of opaque heavy-walled cells termed "replace- 

 ment tissue" by Watson (1948). 



Leaves which have been only slightly differentiated at the time 

 of treatment with 2,4-D show extreme distortions such as the middle 

 leaf in the figure exhibits. In these the leaf is reduced to a bundle 

 of enlarged veins surrounded by sheaths of replacement tissue. In 



