PATHOLOGICAL CONDITIONS 



87 



mineral matter, particularly nitrogen and may interfere in this way with 

 its nutrition as well as through reducing the manufacture and storage of 

 elaborated organic materials. Table 38 shows the mineral constituents 

 of Syringa leaves at the time of defoliation from drought and at the time 

 of normal abscission. The yellowing and dropping of the leaves of 

 dwarf pear trees in times of drought while those of standard trees remain 

 normal is clear evidence that the trouble is due mainly to a lack of 

 moisture, the limited root system of the quince being unable to supply 

 the requirements of the cion in such emergencies."* 



Table 38. — Mineral Constituents of Syringa Leaves at Different Periods 

 IN Percentages of Dry Weight 



{After Soraiier^^*) 



Apparently related to these troubles induced by drought is the tip- 

 burn of certain plants occurring during periods of very high transpiration. 

 Even a few hours of very rapid transpiration in intense sunlight, high 

 temperature and low atmospheric humidity may lead to so great a reduc- 

 tion of the water content in the edges of the leaves of the potato that 

 recovery of turgidity is impossible. ^^ The affected tissues die, a condition 

 known as tip-burn. 



Another closely related form of drought injury has been found on the 

 grape in New York. It is perhaps best described in the words of the 

 original report : 



"Vines affected with the trouble first show a streaked pallidness of the 

 leaves in the intervascular spaces. Later these streaked areas become yellow. 

 The discoloration is more marked near the margins and eventually the pallid 

 areas coalesce and form a yellowed band extending around the margin. As the 

 season advances this band dies and becomes functionless. Isolated areas of the 

 leaf blade deaden and when these join, a considerable part of the leaf tissue may 

 become functionless. When the entire leaf is affected the outer margin often 

 curls upward. The injury is cumulative unless favorable conditions are estab- 

 Ushed in the succeeding years, i.e., optimum rainfall, etc. As a result of the 

 injury to the foliage, growth is materially checked and the wood usually fails to 

 mature well. The fruit does not color nor is the normal amount of sugar fixed. 

 'Shelling' may result. 



