1919] Coit-Hodgson: Abnormal Shedding of Washington Navel Orange 309 



and low humidity, while the former was as equally marked by its rela- 

 tively low temperatures and equableness. The crop in 1917 over the 

 entire state is not estimated to be more than 40 to 50 per cent of that 

 in 1916. 



All the more recent fundamental work in plant physiology has 

 indicated that for plants growing in the open the water relation is 

 the limiting factor. It is at once obvious that under the conditions 

 obtaining in the arid southwest it is the water relation which is most 

 likely to be strained. This is particularly to be considered in connec- 

 tion with the previously mentioned fact that the genus Citrus is 

 undoubtedly of tropical origin and therefore not well adapted by 

 nature to withstand the tremendous water loss incident to the severe 

 climatic complex obtaining under arid conditions. 



Evidence that abnormal water relations due to the influence of the 

 environmental complex may furnish the stimulus to abscission is not 

 lacking. In regard to the cotton plant Balls 39 says: "It is certain 

 that the main factor, if not the only one, is the water-content of the 

 plant." Lloyd, 40 also working with cotton, concludes that "the water 

 deficit is the cause of rise of temperature in the tissues, and this 

 constitutes the stimulus which directly leads to abscission." Howard 41 

 has noted the fact that abnormal water conditions in the soil are 

 immediately shown in the indigo plant, Indigofera arrecta, by leaf-fall 

 or by the shedding of flowers without setting seed. His interpretation 

 of these results will be referred to later. The junior author has already 

 presented data to show that at Edison an abnormal water relation 

 does exist in orange leaves and young fruits during the critical 

 period. 42 He has shown that a daily water deficit of 25 to 30 per 

 cent occurs in the young fruits, which deficit is made up at night. 

 These deficits are at their maxima during the afternoon, at which 

 period the atmospheric pull on the plant for water is at its maximum. 

 A contributing factor to these water deficits lies in the fact that under 

 stress of the tremendous atmospheric pull for water the leaves actually 

 appropriate water from the young fruits. This strain on the plant is 

 not localized but extends throughout the tree. Tensions developed by 

 exterior foliage are transmitted quickly to interior fruits and even to 

 distant roots as was shown by several experiments; for the sake of 

 brevity only one will be described. 



39 Loc. cit., p. 69. 



•*o The Abscission of Flower-buds and Fruits in Gossyjnum, and its Relation 

 to Environmental Changes, Trans. Roy. Soc. Canada, ser. 3, vol. 10 (1916), p. 61. 

 •»i Soil Aeration in Agriculture, Agr. Res. Inst. Pusa, Bull. 61, 1916. 

 42 Loc. cit. 



