ABNORMAL TISSUES. 127 



either spiral or like a bow, and this is visible externally. 

 Damage to shoots in their second or third year is not externally 

 visible. If a frost conies in June, the injured brown wood is 

 joined to the same year's spring-zone of wood by a callus and 

 is followed by subsequent frost-cankers. To an observer 

 without a magnifying glass this might appear to be a double 

 annual ring*. Mayr has noticed such frost-rings since 1890 

 and in Fig. 12 a, b and Fig. 22, drawn from nature, pur- 

 posely has represented some apparent double rings, but their 

 origin has not been yet explained. 



A true double ring occurs only when there are two spring- 

 zones and two summer-zones in an annual zone, in the order, 

 early-wood, late-wood, early-wood, late-wood. This is very 

 rare in forests, but is frequent in towns, where the hot 

 weather and an insufficient water-supply to the roots often 

 cause defoliation at the beginning of August and the growth ter- 

 minates with late-wood. Then leaves reappear and sometimes 

 flowers ; spring-wood is formed and passes into summer-wood 

 at the end of September or in October, at the second leaf-fall 



Hence the formation of spring-wood without a second 

 summer-wood is no double ring, but a reduplication of 

 spring-wood only, when shortly after early spring the tree 

 loses its foliage by frosts, insects, fire, etc., and produces fresh 

 leaves. 



If there is no loss of foliage, but a second crop of leaves 

 appear, especially in oaks when lammas-shoots are produced, 

 or late shoots in September in vigorous young plants, or 2 4 

 series of shoots owing to lopping hedges and shrubs, no 

 irregularity is visible in the w r ood. 



If the anatomical structure of stem-wood is considered 

 n<tnn?d, the wood on the lower side of boughs with very thick 

 cell-walls is abnormal, and so is the root-wood with its very 

 thin-walled cells. The wood of suppressed stems with narrow 

 annual zones is also abnormal and so is the wood of trees 

 with very large zones grown in rich garden-soil. Wood with 

 very thick, folded cell-walls, as on the lower side of boughs, 

 on the eastern side of the root-stock, or on the lower side of 



* 11. Hartijr, " Dnppelringe als Folge von Spiitfrost." Forst Nat, Zeitung, 



