3 10” | 3 tructural | and Physiological Botany. 
form and strongly lignified. The oldest parts of the bundles 
contain narrow vascular cells with annular or spiral thicken- 
ings; the portions formed later having, on the other hand, . ~ 
scalariform cells.!. ‘True vessels occur but rarely. In the — 
Fic. 437.—A lsophila aculeata ; a tree-fern (greatly reduced). 
woody parts of the bundles there are also parenchymatous ~ 
cells containing starch, and the bast-portion is constructed 
1 [The abundance of scalariform cells and scalariform vessels (Fig. 
69, p. 46), is one of the chief characteristics of the woody tissue of — ‘g “ 
nas as contrasted with that of flowering plants, though they are ae Bee 
no means peculiar to this class of plants. Be { 
