SPIRAL STRUCTURES. [ 712 ] SPIRAL STRUCTURES. 



at first (brauclied spiral vessels do occur, 

 more rarely), but ordinarily unite together 

 hj a kind of fusion. The conical extremities 

 overlap to a certain extent (fig. 659) ; 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 adventitious roots generally. 



The above-mentioned confluent spiral 

 vessels pass insensibly into the duds, which 

 are similar confluent rows of cells forming 

 parts of the sohd wood of stems, composed 

 of cells with flat ends applied together. 

 They may resemble in their markings the 

 preceding forms, but in their varied con- 

 ditions form a series leading towards the 

 Pitted Ducts. The scalariform vessels or 

 ducts (fig. 664, and PI. 48. fig. 10), so called 

 from the ladder-like markings, are a very 

 regular form of the reticulated type — this re- 

 gularity appearing to depend, however, upon 

 the relation between the markings of the ad- 

 jacent organs. In the Pitted DucTSwe find 

 the pits only opposite to other pits, therefore 

 on the .sides adj acentto other ducts or to cells ; 

 in the scalariform ducts 

 a spiral-fibrous deposit 

 is conjoined into a net- 

 work by vertical fibres 

 placed opposite the in- 

 tercellular passages or 

 the meeting angles of 

 contiguous cells or 

 ducts, leaving regular 

 slit-lilie spaces opposite 

 the cavities of the adja- 

 cent cells. This form 

 is especially charac- 

 teristic of the Ferns ; 

 but it occurs also com- 

 monly in the Dicoty- 

 ledons in a less regular 



form, passing quite in- Fragments of scalariform 



sensibly into Pitted vessels from a Fern. 



DtrCT.S, as in the wood Magnified 200 diameters, 

 of Eryngium marithnum 

 (PL 48.' fig. 21). The scalariform vessels 

 of Ferns are often slightly luirollable. 



It is mentioned under 1*itted Struc- 

 tures^ also, that a combination of the two 



Fig. 664. 



types sometimes occurs in the same cell, 

 'this is the case in the ducts of the Lime, 

 Mezereon, and other plants (PI. 48. figs. 4, 

 13, & 19). 



Besides the generally diffiised 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 

 mai'kings. The ducts and vessels, indeed, 

 in many cases are formed of vei-y short 

 cellidar 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 Cactacefe, the prosenchyma- 

 tous tissue of the stem presents remarkable 

 spiral and annular cells, in which the fibre 

 becomes so much thickened that it 2>rojects 

 like a riband set with its edge against the 

 cell-wall (PI. 48. fig. 7). The wood of the 

 Mistletoe (figs. 665,666) 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. 48. fig. 4). In 

 the stems of the Leguminosae, parenchy- 

 matous portions occur in the midst of the 

 wood, the cells of which exhibit spiral fibres 

 {Ulex, Sjxivtiuvi). The cellular tissue near 

 the surface of the roots of the epiphytic 

 Orcliids (PI. 48. fig. 6) affords another ex- 

 ample, as also some of the subepidermal 

 cells of the leaves "(fig. 667). The layers of 



Fig. 665. 



Fig. 666. 



Fig. 667. 



Fig. 665. Annular-fibrous cell from the stem of Mistle- 

 toe. Magnified 200 diameters. 



Fig. 666. Cell intermediate between reticulated and 

 pitted, from the Mistletoe. Magnified 200 diameters. 



Fig. 667. Spiral-fibrous cell from the leaf of an Orchid. 

 Magnified 200 diameters. 



cells lining the Anthers of Flowering- 

 plants are characterized by most varied 

 patterns of spiral markings (PI. 40. figs. 1-5) ; 

 in these cells, moreover, we sometimes see 

 the connexion between the fibrous and ho- 

 mogeneous deposits well illustrated, as the 

 cells may have one or more sides marjved 

 with .spiral fibres, while the remainder of 



