SPIRAL STRUCTURES. 



[ 592 ] 



SPIRAL STRUCTURES. 



it is stated that this spiral-fibrous deposit may 

 be taken as the character of a group of struc- 

 tures to be contrasted with those structures 

 described as PITTED, and that the essential 

 distinction in the nature of these two groups 

 lies in the greater extent to which the pri- 

 mary wall is covered in the pitted structures. 

 This is not quite absolute in reference to all 

 spiral-fibrous structures, as in the true un- 

 rollable spiral vessels and similar organs the 

 coils of the spiral fibres are often closely in 

 contact, although not adherent to each other. 

 It has been stated that the various forms of 

 the open spiral, annular, and reticulated de- 

 posits are modifications of the simple close 

 spiral ; but this must be understood only in 

 a morphological sense, since there is no ac- 

 tual change of condition ensuing with age, 

 as has been assumed by some authors, the 

 fibrous layers being always originally depo- 

 sited on the primary wall in the form and 

 pattern which they ultimately possess. There 

 appears to be no real opening of the spirals 

 or breaking up into rings, in consequence of 

 the expansion of the primary wall to which 

 they are attached. 



Fig. 664. 



Fig. 665. 



Fig. 664. Fragments of spiral vessels from the melon. 

 Magnified 200 diameters. 



Fig. 66.5. Magnified diagram of a section of the base of 

 a leaf-stalk arising from a Dicotyledonous shoot, showing 

 the position of the spiral vessels in the leaf-stalk and next 

 the pith of the shoot, the spiral fibres being uncoiled and 

 a little drawn out. 



It will be convenient, in the first place, to 

 speak of the distinct well-marked structures 

 ordinarily known as spiral cells and vessels, 

 occurring in the stems, leaves, &c. of the 



higher plants, before describing certain other 

 forms found in special organs, and to reserve 

 to the end some points relating to the ulti- 

 mate constitution of the secondary mem- 

 branes of cells. Spiral structures are usually 

 divided into true spiral, annular, reticulated 

 and scalariform organs. 



Spiral cells and vessels are perhaps the 

 most generally diffused of the forms. The 

 name spiral vessel is given to elongated cy- 

 lindrical cells tapering to a point at both 

 ends, with a spiral -fibrous deposit lining the 

 primary wall (fig. 664, and PI. 39. figs. 8, 11, 

 12) ; the spiral fibre may be either single, as 

 is most common, double (fig. 664), or a 

 number of fibres may run parallel (Musa, 

 Nepenthes, Zingiberacece, Marantacea). 

 These spiral vessels occur as the first vascu- 

 lar formation outside the pith (MEDULLARY 

 SHEATH) in almost all the Dicotyledons 

 (fig. 665), and as the first vascular formation 

 in the vascular bundles of the stems of 

 Monocotyledons ; also of all other vascular 

 bundles, forming the ribs or veins of petioles, 

 leaves, bracts, sepals, petals, &c. In the in- 

 ternal organs they can only be observed in 

 sections, or when extracted by maceration ; 

 in delicate vessels and petals they may often 

 be observed through the transparent epider- 

 mis. The coiled spiral fibre is mostly elastic 

 enough to bear stretching open like a wire 

 spring ; in this case the primary wall is torn 

 between the coils, and its ragged edges may 

 sometimes be detected. The uncoiled fibres 

 are often seen still unbroken, when a hya- 

 cinth or similar leaf is broken across and the 

 pieces gently drawn apart. Annular vessels 

 closely resemble the preceding, except that 

 the fibrous deposits are in the form of de- 

 tached rings (fig. 666) ; they are the rarest 

 forms ; they are especially remarkable in the 

 Equisetaceas. The reticulated, again, have 

 irregular spiral coils or rings connected more 

 or less by perpendicular or oblique bars (fig. 

 667, and PL 39. fig. 9) into a network ; these 

 two modifications are usually of larger dia- 

 meter than the true spiral vessel, and the 

 reticulated larger (of later origin also in the 

 organs) than the annular. However, mixed 

 forms occur not uncommonly, partly annular, 

 partly spiral or reticulated (fig. 668). They 

 are found in similar situations, but generally 

 do not extend into the more delicate organs. 

 Spiral, annular, and reticulated vessels may 

 be prepared in most beautiful forms and 

 large size from portions of the leaf-stalk of 

 rhubarb, of the stem of the garden balsam, 

 the melon, &c. 



