The Plant Cell 



27 



shells of nuts, are extreme examples. Elongate cells with 

 thickened walls may also be included here, such as bast, 

 or those surrounding the bundles in Indian corn (Fig. 5). 



FIG. 5. Some extreme cell-types : collenchyma (a) ; sclerotic cells from 

 pine needle (6) ; and vegetable ivory (d) ; unusual palisade cell (c) ; bast 

 (e, /) ; prosenchyma (Q) ; from cotton seed-coat (A) ; and stinging cell of 

 nettle hair (i). 



The term " stereome " has been applied to thick- walled 

 cells serving primarily for support. 



20. Tracheids. These are thick-walled cells, more or 

 less elongate, with walls often showing pitted, reticulate, 

 or spiral thickenings. They may possess a considerable 

 lumen, and the mature cell may show no trace of proto- 

 plast. They are usually lignified, and are important in 

 the conduction of water. In many plants (such as the 

 pine and other conifers) they constitute the sole water- 

 conducting system. 



These tissues are likewise important in support, and this 

 fact emphasizes a point worthy of special note, and that is 



