THE FIBRO-VASCULAR BUNDLES. 
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 from 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 ^ \ \ t 
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 septa \ 
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 Coniferae {yide 
Figs. 23, 24, p. 25). There 
is no other difference between these and true vessels ; for vessels with open bordered 
I-'IG. 97. — Tangential longitudinal section through the secondary wood of 
Aüantus s'^andulosa ; g- vessels ; st xylem-rays cut through transversely ; / wood- 
parenchyma ; t tracheVdes ; ^libriform fibres. 
* See Dippel in the Amtlichen Bericht der 39. Vers, der Naturforscher u. Aerzte. 1865 (Glessen), 
PI. 3, Figs. 7-9. Dippel's observations on Cryptogams and the whole 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 Berhn, 1862, p, 448 [Nat. Hist. Rev., 1863, pp. 364-367].) 
