SPIRAL STRUCTURES, 



[ 711 ] 



SPIRAL STRUCTURES. 



It will be convenient, in the first place, 

 to speak of the distinct well-marked struc- 

 tures ordinarily known as spiral cells and 

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

 of the higher plants, before describing cer- 

 tain other forms found in special organs, 

 and to reserve to the end some points re- 

 lating to the ultimate constitution of the 

 secondary membranes 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 

 cylindrical cells tapering to a point at both 

 ends, with a spiral-fibrous deposit lining the 

 primary wall (fig. 659, and PI. 48. figs. 8, 11, 

 12). The spiral fibre may be either single, 

 as is most common, double (fig. 659) ; or 



Fig. 659. 



Fig. 660. 



the internal organs they can only be ob- 

 served in sections, or when extracted by 

 maceration: in delicate vessels and petals 

 they may often be observed through the 

 transparent epidermis. 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 hyacinth 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 detached 

 j rings (fig. 661) ; they are the rarest forms ; 

 they are especially remarkable in the Equi- 

 setaceae. The reticulated, again, have irre- 

 gular spiral coils or rings connected more 

 or less by perpendicular or oblique bars 

 (fig. 662, and PI. 48. fig. 9) into a network. 

 These two modifications are usually of larger 

 diameter than the true spiral vessel, and 

 the reticulated larger (also of later origin in 

 the organs) than the annular. However, 

 mixed forms occur not uncommonly, partly 

 annular, partly spiral or reticulated (fig. 663) . 

 They are found in similar situations, but 

 generally do not extend into the more de- 

 licate organs. Spiral, annular, and reticu- 

 lated vessels may be prepared in most beau- 

 tiful forms and large size from portions of 

 the leaf-stalk of rhubarb, of the stem of the 

 garden-balsam, the melon, &c. 



Fig. 661. Fig. 662. Fig. 663. 



Fig. 659. Fragments of spiral vessels from the Melon. 

 Magnified 200 diameters. 



Fig. 660. 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. 



a number of fibres may run parallel (Mu- 

 ,a, Nepenthes, Zingiberacece, Marantacece). 

 These spiral vessels occur as the first vascu- 

 lar formation outside the pith (MEDULLARY 

 SHEATH) in almost all the Dicotyledons 

 (fig. 660), and as the first vascular forma- 

 tion in the vascular bundles of the stems of 

 Monocotyledons also of all other vascular 

 bundles, forming the ribs or veins of pe- 

 tioles, leaves, bracts, sepals, petals, &c. In 



Fig. 661. Fragment of an annular vessel from the 

 ified 



Melon. 

 Fig. 662 



Magnified 200 diameters. 

 . Portion of a reticulated vessel from the 



Melon. Magnified 200 diameters. 

 3. Frag 



Magnified 200 diameters. 



Fig. 663. 

 the Melon. 



ragment of a spiral and annular vessel from 



Spiral and other vessels are usually simple 



