216 PERIODICITY OF GROWTH 



elements was the mechanical result of the increasing pressure of the bark upon 

 them, whereas Krabbe 1 has shown that the required pressure does not exist. 

 Indeed a very high pressure would be required to produce a noticeable effect. 

 Schwarz z has suggested that a slight increase of pressure might act as a stimulus 

 determining the transition from spring to autumn wood, but since the pressure 

 does not seem to follow a constant rhythm, Schwarz calls a second hypothesis 

 to the aid of the first one by assuming that the plant's power of reaction alters 

 during the summer. 



Hartig 3 and also Wieler 4 consider the change from spring to autumn wood 

 to be the result of altered conditions of nutrition. The former, however, considers 

 the autumn wood, the latter the spring wood, to be the result of better nutrition, 

 while both authors ascribe some importance to the changes in the percentage of 

 water in the stem, which Lutz 5 considers to be of decisive importance. Russow G 

 sought to explain the greater diameter of the cells of the spring wood as being 

 due to their higher turgidity during development, whereas Wieler 7 has shown that 

 the turgidity is no higher in spring than in autumn. 



Haberlandt 8 , Strasburger 9 , and Hartig 10 consider the anatomical differences 

 between the spring and autumn wood to be induced and regulated by the transpira- 

 tion-current, but have brought forward no satisfactory arguments in support of this 

 conclusion. This factor has apparently an accessory importance, for although it 

 can hardly cause the formation and differentiation of the vascular tissue, it may 

 aid in their full development by the influence of use and demand upon growth 

 and supply. 



SECTION 62. Abscission and Leaf-fall. 



In the progress of development many living or dead parts may be 

 thrown off. To the former class belong seeds, spores, and reproductive 

 bodies in general, including pollen and antherozooids n . In Vallisneria the 

 entire male flower is thrown off and ascends to the surface, jso as to render 

 possible the pollination of the floating female flowers. Organs which have 

 fulfilled their function are also frequently thrown off while living or before 



1 Krabbe, Sitztingsb. d. Berl. Akad., 1882, p. 1125 ; Wachsthum d. Verdickungsringes u. der 

 jungen Holzzellen, 1884, pp. 57, 69 (repr. from Abhandl. der Berl. Akad.). 



2 Schwarz, Physiol. Unters. iiber Dickenswachstum und Holzqualitat von Pinus sylvestris, 1899, 



P- 3<>5- 



3 Hartig, Unters. a. d. Forstbot. Inst. zu Miinchen, 1880, 1, p. 148 ; Holz d. Nadelbaume, 1885, 

 pp. 34, 103. 



4 Wieler, Jahrb. f. wiss. Bot., 1887, Bd. xvm, p. 129; Tharandter Forstl. Jahrb., 1892, 

 Bd. XLII, p. 216; 1897, Bd. XLVII, p. 172. 



6 Lutz, Beitr. z. wiss. Bot. von Fiinfstiick, 1895, Bd. I, p. 80. Cf. Jost, Bot. Ztg., 1893, p. 118. 

 ' Russow, Sitznngsb. d. Dorpater naturf. Ges., 1881, p. 41. 



7 Wieler, Jahrb. f. wiss. Bot., 1887, Bd. xvin, p. 80. 



8 Haberlandt, Physiol. Anat., i. Aufl., 1884, p. 371 ; Ber. d. Bot. Ges., 1895, p. 337. 



9 Strasburger, Bau u. Verrichtung d. Leitungsbahnen, 1891, p. 949. 



10 Hartig, Forstl. naturw. Zeitschrift, 1894, III, p. 174. 



11 Another instance is afforded by the division and separation of the cells of bacteria and other 

 asomatophytes. 



