PECAN ROSETTE. 31 



yellowing of the foliage. However, none of the mottling or other 

 signs of rosette appeared at any time during the season. 



Two large, healthy seedling pecans were transplanted to a plat 

 of garden soil containing a great excess of liriie where mortar had 

 previously been mixed for building purposes. These trees have 

 been under observation for three seasons, and no signs of any type 

 of chlorosis have at any time become evident. 



In another small experiment run for two months, 10 seedlings 

 potted in garden soil were given liberal and fairly uniform appli- 

 cations of water throughout the period, while 10 similar potted seed- 

 lings were given only sufficient water to prevent wilting. In the 

 latter case but little growth was made, and a part of the leaves 

 developed a general chlorosis. At no time, however, were any signs 

 of rosette to be seen. 



PROBABLE NATURE OF PECAN ROSETTE. 



In the writer's opinion the pathogenic picture of pecan rosette as 

 shown in the preceding pages, including histological and cytological 

 features, is much more in agreement with the infectious type of 

 chloroses, including the yellows and mosaic groups, than with those 

 chloroses known to be caused directly by soil or climatic conditions. 

 It is not considered, however, that adequate proof has yet been given 

 ■on either side, and the present study is offered merely as one more 

 step in the study of this baffling group of diseases and as a sugges- 

 tion for further research. 



In the environmental type of chlorosis structural changes may 

 occur. The xerophytic tendency is toward a bilateral palisade with 

 restricted air spaces reducing transpiration. Hydrophytic condi- 

 tions produce large, globose cells and loose arrangement of tissues. 

 Excess of sodium chlorid develops enlarged, rounded, spongy, and 

 massive palisade cells and causes reduction in the number of chloro- 

 plastids. Wax coatings are also characteristic of many salt-loving 

 plants. Lack of sufficient water or soil nutrients results in the 

 dwarfing and hardening of the tissues, often followed by general 

 chlorosis but without morphogenic changes. The dimorphism of 

 certain plants when grown under differing environmental conditions 

 is well known, but is in no sense comparable to a diseased or other 

 deranged condition. Changes in metabolism also may be caused 

 directly by soil or climatic conditions. All these changes, however, 

 in the main, are general changes affecting the plant tissues as a 

 whole or tending to affect all similar tissues of the plant alike. 



In pecan rosette profound alterations and derangements in metab- 

 olism, anatomy, and morphogenesis occur together in great com- 

 plexity. Different types of tissue derangement are found in the same 

 leaf. Eeduction and increase in the number of cell divisions, tissue 



