106 ELEMENTS OP STRUCTURAL BOTANY. 



underground stem is called a root-stock or rhizome, and may 

 almost always be distinguished from a true root by the 

 presence of buds. The Solomon's Seal and Toothwort of 



Canadian woods, and 

 the Canada Thistle, 

 are common instances 

 of plants producing 

 these stems. Fig. 137 

 shows a rhizome. 



152. Take now an 

 Onion, and compare 

 it with a Potato. You 

 Fig - 137< will not find any such 



outside appearances upon the former as are presented by 

 the latter. The Onion is smooth, and has no buds upon 

 its surface. From the under side there spring roots, and 

 this circumstance will probably suggest that the Onion 

 must be a stem of some sort. Cut 

 the Onion through from top to bot- 

 tom (Fig. 138). It will then be 

 seen to be made up of a number of 

 coats. Strip off one or two, and ob- 

 serve that whilst they are somewhat 

 fleshy where the Onion is broadest, 

 they gradually become thinner to- 

 wards the top. The long, green tubes 

 which project from the top of the 

 Onion during its growth are, in fact, the prolongations of 

 these coats. But the tubes are the leaves of the 

 plant itself. The mass of our Onion, therefore, consists 



Fig. 137. A rhizome. 



Fig. 138. Vertical section of bulb of the Onion. 



