134 



I'AKT II. THE INTIMATE STEUCTUEE OF PLANTS. 



[29. 



are either pitted with simple or bordered pits, or have annular, 

 spiral, or reticulate, etc., thickenings (see p. 104) ; when fully 

 developed the tissue contains only either air or water. 



The following varieties of this tissue may be distinguished : 

 a. The tracheids, which are closed, generally prosenchymatous, 

 cells (Fig. 95 A JS), occur characteristically in the wood of certain 

 sporophytes (e.g. most Pteridophyta, Coniferaa) and are then com- 

 pletely lignified ; but they also occur elsewhere, as in the tegu- 



FIG. 94. Sclerenchymatous tissue. A The end of a bast-fibre, with strongly thickened 

 pitted walls (longitudinal section x 300). B Wood-fibres from the root of the Cucumber 

 ( k 300), surface view and section. C Fibres from the stem of Helianthus tuberosus ( x 300). 



mentary tissue (velamen) of the aerial roots of certain Orchids, 

 where they are partially lignified; in certain cells of the anther; 

 and in the leaves and cortical tissue of the stem of Sphagnum 

 (gametophyte) where they are not lignified at all (Fig. 95). 



b. The tracheae are true vessels, that is, articulated tubes, the 

 contiguous cell-cavities having been rendered continuous by the 



