410 



The summer oak, Qicerciis peduncidata, Ehrh. behaves somewhat differently. 

 If, in the Spring, a basal growth increases the sclerotic elements, the cover- 

 ing scales show a certain stiffness and remain longer attached to the growing 

 shoot. They thus protect it longer from the dangerous fluctuations in tem- 

 perature. The oak in the warmer Mediterranean countries, Quercus Ilex, L. 

 hardly shows the sclerotic elements in its scantier bud coverings, and some- 

 times they are entirely lacking. In this we are concerned with protection 

 against the summer drought period and find it in the hairs, which develop 

 from the epidermis, and also the cork layer, which develops from the sub- 

 epidermal tissue. 



Before the leaves burst out from the bud, the scales, bent together like 

 a roof, are simply small leaves reduced to stipules, but when the leaves 

 break out, the under side grows further at the base, while the sclerotized 

 outer side does not do so. Consequently the base of the scale, drying from 

 the edges backward, become fleshy, cushion-like and, like a prop, presses 

 the scale outward. This is the time of danger, since even the delicate vege- 

 tative cone is exposed to the fluctuations of temperature, and almost with- 

 out protection. This explains the internal ruptures made by the action of 

 the frost, sometimes found in the spring^ and also the phenomena of 

 shrinking from drought, resulting from constant sharp East winds. 



No matter in what way the protective apparatus of the bud scale is 

 formed in the various species, whether from sclerotic cell layers or from 

 cork layers, layers of hair or masses of resin, the fact holds good that this 

 apparatus develops differently in different years, according to the weather 

 and the amount of nutriment at the time of its formation, and, accordingly, 

 is of different protective power in the following spring. If, for example, 

 the summer has been moist and cloudy, the covering scales tend to develop- 

 ment towards the nature of the green leaf and the cells become larger or 

 less thickened. In spring they react more quickly to the increase of turgor 

 of the tissue and separate from one another more quickly. Thus the grow- 

 ing point is exposed prematurely to inclement spring weather, and so loses 

 too rapidly the protection against its power of transpiration. 



This factor must not be underestimated, for Griiss reports- that, 

 when he removed the strongly developed outer scales from an oak 

 bud, he noticed that the bud was destroyed with great regularity, even 

 if the temperature did not fall and there was present sufficient moisture. 

 Also the inner, more delicately walled coverings became dry since they were 

 not accustomed to the increased transpiration. Uninjured buds kept under 

 similar conditions (on cut twigs) developed further. 



Experiments with beech buds, from which the whole covering had 

 been removed, showed that the young, exposed leaves kept fresh much 

 longer than those of the oak. This is due to the pubescence of the young 

 beech leaves, which protect them from too great transpiration and the con- 



See chapter on the Action of Frost. 

 Log. cit. p. 649. 



