EDGAR TULLIS 



351 



All these statements as to the factors causing leaf casting agree in 

 maintaining that the needles fall because they have become weakened 

 functionally or still are normally weak as a result of the winter's rest. 

 Moreover, the abscission process depends upon the development of the cleav- 

 age layer which presupposes living activity and an increased turgor. Thus, 

 there arises an antagonism ; the leaf organ is not at the time in a condition 

 to function as a normal center of attraction and consumption. Because of 

 its anatomical structure the basal part above the region of the subsequent 

 cleavage can be excited and it is prematurely brought to the development 

 of this cleavage layer by the increase in turgor, which arises in the spring 

 due to exposure to the sun, or has been retained from the previous year, 

 and finds no equalizaion since even the inactive lamina of the leaf do not 

 take up the water from it. This disturbance in the equilibrium of the turgor 

 distribution is the cause of all premature dropping of the leaves. 



In the special case of the pine leaf casting I think that the contrasts 

 described by Ebermayer and, indeed, the sharp contrasts, represent the 

 most frequent cause of the disease. Only in explaining it, I differ from him 

 in so far that I accept as cause the winter's inactivity, evident also in the 

 constitution of the chloroplasts, instead of the excessively increased evap- 

 oration from the needles. Only the base of the needle is excited and de- 

 velops the cleavage laver, which, as will be mentioned under petals, can. 

 under certain circumstances, be produced in an extremely short time. I am 

 of the opinion that the needle does not become dried out, but is put out of 

 action by the cleavage layer. I would like to assume from the absolute 

 scanty elimination of water by pines in winter, that a drying of the needles 

 resulting from an excessively increased evaporation, is not the cause of the 

 discoloration and falling of the needles. An experiment in a water culture 

 of one year old seedlings showed me that a pine ceased its evaporation on 

 the 17th of November despite following days with temperatures of + 3. 4- 7' 

 9 degrees C. Up to the 22nd of December they did not evaporate one gram 

 more of water, although the root stood in water\ It can, therefore, scarcely 

 be assumed that the spring temperature can, in a few days, cause a great loss 

 of moisture, more particularly as the pine is a tree species which evap- 

 orates the least of alP. 



Since the dr}-ing of the needles does not seem to me to be the cause of 

 leaf casting, but rather a lack of equallization in the water supply, resulting 

 from the sharp contrast between the needle surface, weakly assimilative, 

 and its base, already active, I would like to believe the best preventative 

 method to be the avoidance of such sharp contrasts : I, therefore, add the 

 proposals made by Ebermayer : — 



A. Increase in soil temperature : (i) due to the prevention of too great 

 cooling during the winter by means of leaf, brush or moss coverings; (2) 



1 Sorauer, Studien iiber Verdunstung-. Forschungen auf d. Gebiete der Agri- 

 kulturphysik. Vol. Ill, Parts 4 & 5, p. 10. 

 " V. Hohnel, loc. cit. Vol. II, p. 411. 



