358 



The abscissed branches of oaks' should be mentioned. In spruces 

 except for the twigs frequently found bitten off by squirrels-, there are 

 probably no actual abscissed twigs. 



Further, this phyllocladia, or loosening of the branches, has been ob- 

 served in Xylophylla and Phyllocladus^, in all Dammara species and 

 especially in Dammara australis, according to A. Braun, in several Podo- 

 carpus species, in Guajaceae, Piperaceae, many bushy Acanthaceae, in 

 Laurus Camphora, Crassula arborescens, Portulacaria afra, Taxodium 

 distichum*, in Tilia" in UUnus pendula, Evonymus, Prunns Padus, Erica, 

 Salix"*, etc. 



The trees partially owe their characteristic habit of growth to these 

 abscissed twigs. But the process of freeing varies according to the habitat, 

 weather and other agencies. Thus Rose, for example, emphasizes that, 

 with continued drought, the branches fall more abundantly; in the majority 

 of cases, side shoots are dropped, but many plants lose their tips as well. 

 The last case is observed most frequently in young trees grown on fertile 

 soil. Nordlinger" emphasizes that predominantly the weakly grown 

 branches are the ones shed. 



Just as we find the leaves falling in summer, we also tind a summer 

 abscission of the branches. Gymnocladus, Catalpa bignonioides, Gleditschia, 

 Tilia and especially Ailanthus glandidosa exhibit the same formation of an 

 abscission layer and the separation of the cells from one another as found 

 in the case of fallen leaves. In young shoots of Ailanthus it may be ob- 

 served that, besides the parenchyma, even the still unlignifted cells of the 

 vascular bundles are involved in the formation of the cleavage layer. No 

 cork is developed at this time either near the abscission or in the upper 

 surface of the bark of the branch. Hence we often find it affirmed that the 

 process of abscission does not depend upon the formation of a cork layer 

 and that this cork layer is to be considered only as a protective layer for 

 the free-lying parenchyma appearing sometimes earlier (before the cleav- 

 age), sometimes later. 



We owe very extensive investigations of twig abscission to v. Hohnel', 

 who has included conifers especially in the scope of his work, and has come 

 to the conclusion that, in them, one cannot speak of any twig abscission, 



1 Th. Hartig, Naturgeschichte d. Forstl. Kulturpflanzen, p. 119. Pfeil, Deutsche 

 Holzzucht, 1860, p. 136. Wigand, Der Baum, 1854, p. 67. Schacht, Der Baum, 1853. 

 p. 305. Lehrbuch d. Anatomic usw., 1859, II, p. 19. 



2 Ratzeburg, Waldverderbnis, I, 1866, p. 219 (Plate 28, Fig. 3). Compare Beling 

 and further Roth (tJber Absprunge bei Fichten), Bot. Jahresbericht von Just, II, 

 p. 968, 971, and v. Hohnel, Bot. Jahresb. VI, Gonnermann, tJber die Abbisse der 

 Tannen and Fichten. Bot. Zeit. von v. Mohl and Schlechtendal, 1865, No. 34. RosSi, 

 Bot. Zeit. 1865, No. 41. 



a v. Mohl, tJber den Ablosungsprozess saftiger Pflanzenorgane Bot. Zeit. 1860, 

 p. 274 and 275. 



4 Rose, tlber die "Absprunge" der Baume. Bot. Zeit. 1865, p. 109 (No. 14). 



5 V. Mohl, tJber den Ablosungsprozess saftiger Pflanzenorgane Bot. Zeit. 1860, 

 p. 274 and 275. 



Nordlinger, Deutsche Forstbotanik. 1874, I, p. 199. 



" V. Hohnel, tJVjer den Ablosungsvorgang der Zweige einiger Holzgewachse und 

 seine anatomischen Ursachen. Mitteilungen aus dem forstlichen Versuchswesen 

 Oesterreichs von v. Seckendorff, III, 1878, p. 255. Weitere Untersuchungen iiber den 

 Ablosungsvorgang von verholzten Zweigen. Bot. Centralbl. 1880, p. 177. 



