ACROGENS. 53 



They vary in size, from the Tree-ferns of the tropics, 

 which may be fifty or sixty feet high, to the delicate 

 Maiden-hair fern of the shady dell. In temperate 

 climes ferns have usually a simple or branched un- 

 der-ground stem, called a rhizome, a root-stalk, from 

 which grow root-hairs and fronds. The epidermis of 

 the stem is of brownish hue, and when young and 

 above ground is provided with stomata. Chap. VI, 

 Sec. 6. As in higher plants, the general cellular 

 structure consists of many-sided cells, containing 



Fig. 23. 



chlorophyll and starch granules. There are also ves- 

 sels, (annular, spiral, and scalariform, or ladder-like,) 

 and fibrous or woody tissue, together forming the 

 harder tissues. 



Fig. 23 illustrates the growth of a fern at the 

 summit, together with the metamorphosis of the ter- 

 minal cell into the various tissues. 



In flowering plants the terminal cell of the leaf -bud 

 becomes barren, and the enlargement of the leaf de- 

 pends on the multiplication and growth of cells 



