SPIRAL STRUCTURES. 



[ 593 ] 



SPIRAL STRUCTURES. 



Spiral and other vessels are usually sim- 

 ple at first (branched spiral vessels do occur 

 more rarely), but ordinarily unite together 



Fig. 666. 



Fig. 667. Fig. 668. 



Fig. 666. Fragment of an annular vessel from the 

 melon. Magnified 200 diameters. 



Fig. 667. Portion of a reticulated vessel from the me- 

 lon. Magnified 200 diameters. 



Fig. 668. Fraement of a spiral and annular vessel from 

 the melon. Magnified 200 diameters. 



by a kind of fusion ; the conical extremities 

 overlap to a certain extent (fig. 664), and 

 thus the articulation is more or less oblique. 

 This fusion is much more evident and com- 

 plicated in roots, rhizomes, and abbreviated 

 stems, than in stems with developed inter- 

 nodes. The elementary cells are then gene- 

 rally much shorter, and the vessels formed 

 from them branch out in various directions 

 through the tissue. This is very well seen in 

 the roots of many herbaceous plants, such as 

 the dandelion, chicory, &c., and at the point 

 of origin of the vascular bundles of adventi- 

 tious roots generally. 



The above-mentioned confluent spiral ves- 

 sels pass insensibly into the ducts, which are 

 similar confluent rows of cells forming parts 

 of the solid wood of stems, but composed of 

 cells with flat ends applied together. They 

 resemble in their markings the preceding 

 forms, but in their arrangement and consti- 

 tution they are closely associated with the 

 PITTED DUCTS. The scalariform vessels 

 or ducts (fig. 669, and PI. 39. fig. 10), so 

 called from the ladder-like markings, are a 

 very regular form of the reticulated type ; 

 this regularity appearing to depend, how- 

 ever, upon the relation between the mark- 

 ings and the adjacent organs. In the PITTED 

 DUCTS we find the pits only opposite to other 

 pits, therefore on the sides adjacent to other 

 ducts or to cells ; in the scalariform ducts a 

 spiral-fibrous deposit is conjoined into a net- 



Fig. 669. 



Fragments of scalariform 

 vessels from a Fern. 



Magnified 200 diameters. 



work by vertical fibres placed opposite the 

 intercellular passages or the meeting angles 

 of contiguous cells or ducts, leaving regular 

 slit-like spaces oppo- 

 site the cavities of the 

 adjacent cells. This 

 form is especially cha- 

 racteristic of theFerns, 

 but it occurs also com- 

 monly in the Dicoty- 

 ledons in a less re- 

 gular form, passing 

 quite insensibly into 

 PITTED DUCTS, as in 

 the wood oiEryngium 

 maritimum(PL 39. fig. 

 21). The scalariform 

 vessels of Ferns are 

 often slightly unrel- 

 iable. 



It is mentioned un- 

 der PITTED STRUC- 

 TURES, also, that a 

 combination of the two 

 types sometimes occurs in the same cell. 

 This is the case in the ducts of the Lime, 

 Mezereon, and other plants (PI. 39. figs. 4, 

 13 & 19). 



Besides the generally diffused spiral and 

 other vessels and ducts above described, 

 cells, properly so called, that is, such as 

 never become elongated very greatly in one 

 particular direction, belonging to particular 

 organs and plants, present the same kind of 

 markings. The ducts and vessels, indeed, 

 in many cases are formed of very short cel- 

 lular elements; but these may be distin- 

 guished from proper cellular tissue charac- 

 terized by spiral secondary deposits. Under 

 this head may be cited first certain wood-cells. 

 In the Cactacese, the prosenchymatous tissue 

 of the stem presents remarkable spiral and 

 annular cells, in which the fibre becomes so 

 much thickened that it projects like a ri- 

 band set with its edge against the cell-wall 

 (PI. 39. fig. 7). The wood of the Misletoe 

 (figs. 670, 671) also exhibits spiral-fibrous 

 cells ; that of the Yew (TAXUS) is composed 

 of true spiral-fibrous cells, and others with 

 bordered PITS and an internal spiral-fibre 

 in addition (PI. 39. fig. 4). In the stems of 

 the Leguminosae parenchymatous portions 

 occur in the midst of the wood, the cells of 

 which exhibit spiral fibres (Ulex, Spartium). 

 The cellular tissue near the surface of the 

 roots of the epiphytic Orchids (PI. 39. fig. 6) 

 affords another example, as also some of the 

 subepidermal cells of the leaves (fig. 672). 



