28] 



CHAPTER II. THE TISSUES. 



93 



5. Tracheal tissue consists of cells which early lose their proto- 

 plasm ; their cell-walls are generally, but not always, lignified, and 

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

 spiral, or reticulate, etc., thickenings (see p. 74) ; when fully 

 developed the tissuejjontains only either air or water. 



The following varieties of this tissue may be distinguished : 



FIG. 72. Sclerenchymatous tissue. A The end of a bast-fibre, with strongly thickened 

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

 ( x 300), surface view and section. C Fibres from the stem of Helianthus tu^crouus ( x 300). 



a. The tr 'ache ids, which are closed, generally prosenchymatpus 

 cells (Fig. 73 J5), occur characteristically in the wood of certain 

 plants (e.g. most Pteridophyta, Coniferse) and are then completely 

 liguified ; but they also occur elsewhere, as in the tegumentary 

 tissuB (velamen) of the aerial roots of certain Orchids, where 

 they are partially lignified ; in certain cells of the anther-wall ; 



