359 



so long as the shedding of Uving, fresh and sappy branches is meant by it. 

 In conifers, the branch to be shed first dies on the trunk, becoming yellow 

 or brown ; it is shed in the usual way only after death, and a cork layer is 

 always formed ; in this process, the wood breaks off at a definite place. 

 The abscissed twigs of deciduous trees are shed in a living and sappy con- 

 dition by means of a parenchyma zone traversing the thick wood but with- 

 out the assistance of a cork layer. 



The age of normally abscissed twigs varies greatly. In Taxodium they 

 are always one year old; in Pinus strobus, always three years old; in Finns 

 Larcicio, 2 to y year old ; in Pinus silvestris, 2 to 6 years old ; in Thuja 

 occidentalis, 3 to 11 years old. It was stated at the outset that spruces and 

 firs are said not to shed their branches. Nevertheless, I remember once 

 having seen fresh spruce shoots with a dismembered surface resembling 

 an articulation. 



In deciduous trees, it can be seen clearly that the twigs usually shed 

 are those grown from lateral buds or adventitious eyes which are often 

 weakly, and have grown only to short shoots. Only in poplars and willows 

 and seldom in oaks are long shoots abundantly shed, and then only older 

 ones (branches up to 6 years old). In rare cases the. process is observed 

 also in Pnimis Padus and Evonymiis europaea, while in other trees usually 

 one year old shoots alone are shed. 



Worthy of our attention is v. Hohnel's observation that the wood of 

 Thuja occidentalis is weaker where the constriction will appear later, than 

 at any other place. At the place which will later be the cleavage surface, 

 ihe wood is greatly constricted. The parenchyma cells of the bark enlarge 

 so that a considerable loosening is produced. In Thuja orientalis the fleshy 

 branch cushion is lacking, and no regular shedding is found. Meehan^ 

 found in Ampelopsis quinque folia that the basal internode remains stationary 

 and, in the following year, produces new shoots, which in turn disarticulate 

 with the occurrence of colder weather. 



The law formulated for leaf casting may be applied to abscissed 

 twigs : — the centre of consumption, which here is the twig, for some reason, 

 no longer forms the normal centre of attraction for the undiminished flow 

 of water and an excess of water accumulates accordingly in the basal zone 

 which is still capable of reaction, and anatomically differently constructed. 

 Either the branches, from the beginning, have been more weakly set, or, 

 because of an unfavorable habitat they do not develop so far or, in great 

 summer drought, they have become prematurely ripe or they are rendered 

 incapable of action by cold, etc. In a weak organ, the relative excess of 

 water makes itself felt first at the base. If this organ develops, from the 

 start, with the presence of a large water supply, no shedding takes place. 

 Wet years exhibit little if any twig abscission. The theory held by fores- 

 ters, that years with much twig abscission initiate good seed years, has its 



1 Meehan, On disarticulating branches in Ampelopsis. From "Proceed, of the 

 Americ. Acad, of Philadelphia." Part I, 1S80, im Bot. Centralbl. 1880, p. 1005. 



