THE STEM. 



25 



productive structure belonging to the leaf-scale region of the stem 

 is the tuber (fig. 19). which consists of a fleshy thickened subter- 

 ranean axis, arising in the axil of a leaf-scale, having its own in- 

 ternodes considerably developed, so that its leaf-scales are scattered 

 and cover isolated buds or " eyes." Tubers of analogous character 

 are sometimes formed from aerial branches, as in many epiphytic 

 Orchids, where they have a green colour and are known as pseudo- 

 bulbs. 



Bulb. The bulb (fig. 16) is a stem remaining permanently in 

 the condition of a bud. Its axis consists of a disk or short coni- 

 cal plate, from the upper surface of which arise leaf-scales of 

 fleshy character more or less overlapping each other and enclosing 

 the points of growth, while one or more circles of adventitious 

 roots are given off from the base (fig. 17). Bulbs are named, ac- 



Fig. 16. 



Fig. 17. 



Scaly bulb of Lilium candidum, 

 adventitious roots. 



Tunicated bulb of the Garden Hyacinth, cut 

 through perpendicularly, showing the leaf- 

 scales arising from the abbreviated stem 

 (6), and the young bulbils or cloves (a, a) 

 formed in the axils of leaf-scales. 



cording to the character of their leaf-scales, scaly or squamose when 

 these only partia lly overlap (Lily) , and tunicated when the scales form 

 complete sheaths (Onion, Hyacinth). Bulbs produce flowering 

 axes either from the terminal or from axillary buds. They are 

 multiplied by buds developed in the axils of the scales in the form 

 of new bulbs (fig. 17, a, a), which sooner or later become detached. 



