44 BRANCH- CASTING [CH. 



be aided or intensified by external factors — e.g. the Oak 

 often casts numerous twigs after a hot summer, and the 

 fact that large quantities of starch may be shed in these, 

 and that the tree may bear a heavy crop of acorns the 

 same autumn, invalidates any belief that the casting is 

 simply due to starvation or weakness, although it appears 

 to be true that drought, age, and especially feebleness of 

 growth on poor soils exert their effects; in the Oak it is 

 chiefly weak lateral shoots which are first cast. Young 

 trees on good deep soil often defer the casting for many 

 years. Indeed it is the rule that young Oaks, and 

 Limes, Planes, Hazel, &c., do not cast branches normally ; 

 but even vigorous young Oaks and Limes may thus cast 

 their terminal shoots. 



The amount and kind of branches thus cast, and the 

 position of the planes of separation (absciss-layers) on the 

 shoots, vary in different species. For instance the Scots 

 Pine casts needle-bearing dwarf-shoots several years old, 

 and it may be taken as a rule that it is dwarf-shoots 

 which are cast by most trees. But the latter statement 

 expresses no universally applicable rule, for Poplars and 

 Willows cast long-shoots from one to six and even more 

 years old, three feet or more in length, and the Oak 

 may shed twigs two feet long. Even the inflorescences 

 may be cast with the current year's shoot in Willows and 

 Prunus Padus. 



In Oaks and Poplars the position of the absciss-layer 

 is at the base of the cast shoot, but in W^illows and Pru7ius 

 it may be situated above the lowermost undeveloped 

 leaves. 



That this curious phenomenon must play a part in 

 modifying the shape and symmetry of the branch-system 

 or crown of the tree is obvious. 



It has been pointed out that the very loose structure 



