Oct. 14, 1918 Hardening Process in Plants 89 



the spots until the growth of the intumescences began. At about the 

 second or third day after freezing, the spotted areas showed more or less 

 irregularly an increase in the peroxidase reaction. In young tumors 

 which were growing rapidly there was a marked increase in the peroxi- 

 dase reaction of the tumors over the rest of the leaf, with the exception 

 of the veins. Plate A gives an idea of the relative peroxidase reaction 

 shown by the purple color in the tumors as well as the relative color of 

 tumor and normal areas of the leaf before applying the reaction. Ih 

 all cases leaves which were plunged into boiling water for two minutes 

 gave negative tests for peroxidase. 



Woods (^o) found greater amounts of peroxidase in the spotted areas 

 of tobacco leaves infected with mosaic than in the normal areas of the 

 leaf. He attributed the lighter color to the oxidation of the chlorophyll 

 by the abnormally increased peroxidase. He submitted evidence to 

 show that chlorophyll is destroyed by peroxidases in vitro. It is possible 

 that there is a similar connection in the case of tumored cabbage leaves 

 and that this accounts for the lighter color of the intumescences. 



These abnormal growths were first obtained in the winter of 191 5 in 

 great abundance and at will. The only conditions necessary are to have 

 the temperature to which the cabbages are exposed so regulated that the 

 freezing can be stopped before the leaf is killed by being frozen through- 

 out. The temperature required for cabbages is about — 3°C., with an 

 exposure of about 30 minutes. In June, 191 6, Smith (45) began some 

 experiments on the production of abnormal growths by injecting certain 

 simple chemical substances or exposing plants to the vapor of these 

 substances. This interesting work has already been reported. The 

 chief point in his report to be noticed here is that a number of these sub- 

 stances, ammonia, acetic acid, amins, etc., are either acid or alkaline in 

 reaction, and some strongly so, a point which will be discussed later. 

 Among other plants tested, cauliflower plants (BrasHca oleracea hoirytis) 

 were exposed to ammonia vapor. The intumescences which resulted 

 might well be taken for those shown in the illustrations in this paper, so 

 similar are they. The differences seem to be that on exposure to ammo 

 nia a smaller area is affected, such as that about a stomatal opening. 

 In Plate 63 of Smith's report {45) the growths appear along the veins in 

 abundance. A similar stimulus can be given by freezing, a purely 

 physical process at the start, as well as by chemical treatment, indicating 

 a similar basic cause in the two processes. 



Intumescences of this kind are quite commonly found on cabbage, 

 resulting from aphis punctures and from breaking the leaf surface; injec- 

 tions of water which loosen the epidermal layer produce them; in fact, 

 it is only necessary to puncture the epidermis to cause their development. 

 Hence, it seems that the growth response is a common phenomenon in 

 cabbage and that it may be caused by a condition common either to 



