UNDERGROUND STEMS 181 



rhizome elongates horizontally under the surface of the ground, 

 neither seeking the light nor growing away from the earth. 



Rhizomes grow best at certain depths in the soil, and, if the 

 depth is changed by adding or removing soil from over them, they 

 will grow up or down until the required depth is reached. By a 

 covering of manure or straw, the rhizomes of Quack Grass and 

 some other weeds may be induced to grow to the surface or even 

 out of the ground. Such weeds are sometimes eradicated by 

 removing the covering and exposing the rhizomes to drying and 

 freezing after they have been induced to grow 

 to the surface. 



Rhizomes elongate and push forward 

 through the soil by growth at one end. It 

 is near this growing end that the aerial por- 

 tions are produced from season to season. 

 As the rhizomes push forward, the older por- 

 tions behind die away, and if the rhizome is 

 branched, as many of them are, the branches 

 become separated and form independent 

 rhizomes. The creeping and branching habits 

 of rhizomes are important features for vege- 

 tative propagation. Rhizomes are able to 



creep through a soil which is already well ,. IG * , ' ^ 



^ J section of an Onion 



occupied by other plants, and consequently, above and i en gthwise 



plants having rhizomes are able to spread section below, c, main 

 where there is no chance for seed to develop, bud; 6, small buds; s, 



The tuber occurs among plants where cer- stem ; r > roots; /, fleshy 

 tain regions of the undergound stem or its 

 branches become much enlarged in connection with food storage. 

 The most familiar tuber is the Irish Potato. The nodes are 

 marked by the scale-like leaves in the axils of which occur the 

 small buds or eyes. The presence of nodes identifies the Potato 

 tuber as a stem structure. It is the stem portion of tubers that 

 is prominent, the leaves and buds being small. Another tuber 

 with nodes more prominent than in the Irish Potato and also of 

 some value for food is the Jerusalem Artichoke. 



In bulbs the leaves or leaf bases are more prominent than the 

 stem, which is short, erect, and enclosed by the leaf structures. 

 Most of the food is stored in the leaf structures rather than in the 

 stem. Some common bulbs are those of the Onion, Lily, Hya- 



