THE FIBRO-VASCULAR BUNDLES. 



IJ7 



diately into the scalariform markings. The detachment of the spiral band of the first- 

 formed spiral vessels in stems and petioles which are growing rapidly, appears to 

 depend solely on the separation of the band frorri the thin quickly-growing wall 

 common to the vessel and to the adjoining cells. If the band were unrolled 

 owing to the absorption of this wall, the adjoining cells would necessarily be opened. 

 If the septa of the separate vascular cells are very oblique, the cells assume a 

 prosenchymatous appearance, and the more this is the case the more does the vessel 

 appear completely continuous. In the xylem of Ferns this is often carried to so 

 great an extent that, after 

 the cells have been isolated 

 by maceration, it would be 

 easy to believe that we 

 have not cells united into 

 vessels, but fusiform pros- 

 enchyma (Fig. 27, p. 27); 

 but in this case also all kinds 

 of transitions occur to the 

 typical scalariform septal 

 Vessels with prosenchyma- 

 tous constituents form the 

 immediate passage to the 

 vascular wood-cells (Tra- 

 cheides). If the form of 

 the cells is such that there 

 is no difference between 

 the longitudinal wall and 

 the septum — which is pos- 

 sible only in decided pros- 

 enchyma — then the perfor- 

 ations between cells which 

 lie above and cells which lie 

 beside one another are no 

 longer different in form ; 

 rows of cells no longer 

 give rise to continuous 

 tubes, but whole masses of 

 cells (bundles, &c.) are con- p \ 

 nected with one another by 

 means of open bordered 

 pits. This occurs in an 

 especially marked manner 

 in the tracheides in the 

 wood of Goniferae {nj'ide 

 Figs. 23, 24, p. 25). There 

 is no other difference between these and true vessels ; for vessels with open bordered 



Fig. 97. — Tangential longitudinal section through the secondary wood of 

 AilaiUus glandiUosa; g g vessels ; st xylem-rays cut through transversely ; / vvood- 

 parenchyina ; t tracheides ; (/"libriform fibres. 



' See Dippel in the Amtlichen Bericht der 39. Vers, der Naturforscher u. Aerzte. 1865 (Giessen), 

 PI. 3, Figs. 7-9. Dippel's observations on Cryptogams and the v^'hole description of the formation 

 of vessels here given, their passage into tracheides, and especially the fact that the air-conducting 

 vascular forms have open bordered pits, and are thus in communication even when the paren- 

 chymatous constituents of a vessel are united not by large openings, but by narrow fissures, &c. (and 

 are hence not closed cells, as Caspary thinks), compel us to reject Caspary's hypothesis of the 

 absence of vessels in Cryptogams and many Phanerogams. (See Caspary, Monatsberichte der k. 

 Akademie der Wissenschaften in Berlin, 1862, p. 448 [Nat. Hist. Rev., 1863, pp. 364-367].) 



