1 68 Journal of Agricultural Research VOI.XXI.NO.S 



This difference between the first and second season's growth of the 

 same plants is thought to have some bearing on the character of the asso- 

 ciation between chlorosis and lack of vigor. Three possible explanations 

 of this association present themselves: (i) A general lack of vigor may 

 predispose to chlorosis; (2) the conditions which cause reduced vigor at 

 this nursery may also favor chlorosis, so that the two phenomena are 

 coordinate effects of the same set of conditions; (3) the poor growth of 

 chlorotic plants may be a result of the chlorotic condition. 



The first of these possibilities has little to recommend it from the the- 

 oretical standpoint, in so far as vigor is judged by the growth rate of the 

 tops. Sachs (rp) finds iron-hunger chlorosis especially common in rap- 

 idly growing shoots. The evidence in Tables III and IV that the chlo- 

 rotic seedlings were not seriously deficient in growth of tops at the begin- 

 ning of their second season tends further to discredit this first suggested 

 explanation. Such difference in the primary leaf system as did exist 

 between the different subclasses was more likely the effect of chlorosis 

 during the preceding year than the cause of chlorosis during the current 

 season. However, the root weights are of some interest. That poor root 

 development may predispose to chlorosis is entirely probable. In a soil 

 deficient in available iron, a plant with little root surface would presum- 

 ably be especially liable to iron hunger. 



The second and third explanations are both believed to apply in part 

 to the obvious association of chlorosis and poor development. Lack of 

 balance of soil solution around the roots of the plant sufficient to interfere 

 seriously with the chlorophyll-forming function might easily interfere 

 more or less with some of the other processes in the plant to an extent 

 sufficient to decrease growth rate. Furthermore, the shortage of chloro- 

 phyll and consequent decreased ability to synthesize carbohydrates 

 would very naturally result in decreased growth of at least some parts of 

 the plant. It is, therefore, the belief of the writers that poor develop- 

 ment of roots (but not of tops) is probably a contributory cause of chlo- 

 rosis, that both chlorosis and poor development of the whole plant may 

 in part be parallel and independent results of excessive lime, and that 

 chlorosis is almost certain to result in a still further decrease in growth. 

 A circular relation, therefore, seems to obtain. 



The data on Douglas fir in Tables III and IV indicate that with this 

 species, as with the western yellow pine, there is a decided difference in 

 growth between green and chlorotic seedlings. With this species as with 

 the other, the weight of the roots of the chlorotic seedlings is much more 

 deficient than the length. From the data it is not possible to determine 

 separately the first and second year's growth, as was possible to a certain 

 extent in the western yellow pine. The deficient terminal bud produc- 

 tion and development on the part of the chlorotic plants is as evident 

 in Douglas fir as in western yellow pine. 



