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 aérial 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 Mee ec capiseed 
creep through a soil which is already well 33 of an Onion 
occupied by other plants, and consequently, above and lengthwise 
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; 7 roots; f, fleshy 
: 3 -,_ scales. After Andrews. 
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- 
