254 



FUNDAMENTALS OF FRUIT PRODUCTION 



of periodic drought, it is probable that leaf -fall results directly from the desicca- 

 tion incident to the increased transpiration and decreased absorption during the 

 dry period. Autumnal leaf-fall in cool climates probably is due to desiccation 

 resulting from continued transpiration at a time when absorption is diminished 

 by reason of low temperature, although desiccation due to dryness in the soil or 

 air may cause the absciss layer to develop in early summer. A severe frost in 

 early autumn may retard leaf-fall through injury to the tissues that develop the 

 absciss layer. 



"The shedding of leaves at the inception of a cool or dry period is of inestima- 

 ble advantage, especially in trees with delicate leaves, because of the enormously 

 reduced transpiration thus resulting. The leafless tree is one of the most per- 

 fectly protected of plant structures, since impervious bud scales and bark cover 

 all exposed portions." 



Accompanying leaf-fall the moisture contents of the various tissues 

 change. From summer to early winter there is a considerable lowering 

 of the moisture percentage as shown in Table 11, adapted from data by 

 Baake et al.,^ showing the moisture percentage in twigs of several varieties 

 of apple. 



Table 11. — Percentage Moisture Content of Apple Twigs 



Dormant 



Bud 

 swelling 



Blossom- 

 ing 



Summer 

 growth 

 period 



Wood 

 ripening 



Hibernal 



Oldenburg 



Wealthy 



Yellow Transparent . . . . 



Mcintosh 



Red Astrachan 



Jonathan 



Winesap 



Grimes 



Ben Davis 



Average, 17 varieties 



42.43 

 45.64 

 45.04 

 46.32 

 46.82 

 47.30 

 43.52 

 47.58 

 48.23 

 47.76 



45.765 



48.65 

 53.31 

 51.66 

 52.10 

 50.52 

 54.86 

 52.00 

 50.57 

 49.54 

 51.94 



65.53 

 62.15 

 65.57 

 61.87 

 65.79 

 62.62 

 64.48 

 65 . 22 

 63.20 



58.98 

 60.50 

 61.11 

 61.49 

 57.76 

 60.82 

 58.77 

 58.67 

 58.95 

 59.44 



52.56 



64.19 



58.92 



53.67 

 52.98 

 55.04 

 51.88 

 55.31 

 51 . 63 

 51.53 

 54.16 

 51.09 



52.55 



INCREASING HARDINESS 



By Increasing Sap Density. — A logical consequence of the theory of 

 death through withdrawal of water by freezing is the correlation of an 

 increased sap density (molar concentration) with a lower killing tem- 

 perature for any given species. Chandler's investigations have demon- 

 strated this. Sap density was increased by various means, such as with- 

 holding water, watering with mineral solutions, inducing absorption of 

 various substances and it was reduced by shading; in each case with 



