THE FIBRO -VASCULAR BUNDLES. 



99 



zontal walls are often entirely absorbed, or they have large round cavities. The more 

 oblique the septum, the more do the perforations take the form of narrow broad 

 parallel fissures ; and the thickening-bands of the septum which remain present more 

 or less the appearance of rungs of a ladder, while reticulated combinations of them are 

 often formed. The scalariform septum is found, according to Sanio, not only in reti- 

 culately thickened vessels and those with bordered pits, as was previously supposed, 

 but also in spiral vessels {e.g. in Gasuarina, Olea, 

 Vitis) where turns of the spiral band pass im- 

 mediately into the scalariform markings. The 

 loosening of the spiral band of the first-formed 

 spiral vessel in stems and leaf-stalks of rapid 

 growth, appears to depend solely on the loosen- 

 ing of the band from the thin quickly-growing 

 wall which is common to the vessel and the ad- 

 joining cells. If the band could be unrolled 

 owing to the absorption of this wall, the adjoin- 

 ing cells must necessarily be opened. If the septa 

 of the separate vascular cells are placed very ob- 

 liquely, the latter assume a prosenchymatous 

 appearance (Fig. 85), and the more this is the 

 case the more does the vessel appear discon- 

 tinuous. In the xylem of Ferns this is often car- 

 ried to so great an extent that, after isolation 

 of the single cells by maceration, it would be easy 

 to believe that it is not the remains of vessels, 

 but fusiform proscnchyma that is left (Fig. 29); 

 but here also all kinds of transitions occur to the 

 typical scalariform septa ^ Vessels with prosen- 

 chymatous constituents now form the immediate 

 passage to the vascular wood-cells (Tracheides). 

 If the form of the cells is such that there is no 

 longer any difference between the longitudinal 

 wall and septum, which is possible only in deci- 

 dedly prosenchymatous forms, then the perfora- 

 tions of the cells which lie above and next one 

 another are no longer different in form ; rows 

 of cells no longer arise in an especially marked 

 manner resembling continuous tubes, but whole 

 masses of cells (bundles, &c.) are connected "with 

 one another by means of open bordered pits. 



This occurs in an especially marked manner in the tracheides in the wood of Coniferae 

 {vide Figs. 25, 26, p. 25). There is no other difference between these and true vessels ; 

 for vessels when they have bordered pits behave in reference to the side-walls exactly 

 like tracheVdes when they have open bordered pits (Fig. 27). The separate elements 



Fig. 85. — From the very young fibro-vascular bundle 

 of a young leaf-stalk oi Scrofi/uilaria aquatica ; part 

 of a spiral vessel surrounded by procambium ; two 

 spirally thickened cells are in prosenchymatous appo- 

 sition ; by the elongation of the leaf-stalk the turns of 

 the spiral band now lying close to one another are 

 drawn apart ; the spiral band becomes detached from 

 the thin wall which is common to the vessel and to 

 the adjoining cells, and so a spiral band is formed 

 capable of unrolling. 



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

 Feb. 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 

 tracheal fonns have open bordered pits, — even when the parenchymatous constituents of a vessel are 

 united not by large cavities, but by narrow fissures, &c. or are connected with one another (and 

 are hence not closed cells, as Caspary thinks), — compel us to consider as erroneous Caspary's sup- 

 position of the absence of vessels in Cryptogams and many Phanerogams. (Cf Caspary, Monats- 

 herichte d. k. Akademie der Wissenschaften in Berlin, July 10, 1862.) 



