85 CLASS-BOOK OF BOTANY. 
th 
quently they are very much thickened, and serve as stores 
of nutritive material for the plant, just as some roots do. 
Ginger is the dried rhizome of a tropical plant, and furnishes 
a good example. In some plants the rhizomes are greatly 
elongated, and creep for a great length in the soil, as is the 
Fig. 173. Hydrocotyle asiatica, showing the creeping rhizomes. 
case in many sea-side and swamp plants—e.g., Mimulus ‘e- 
pens, Convolvulus soldanella, &. Plants having such rhizomes 
are often of great service for binding loose sand, but in culti- 
vation they frequently become dreadful pests. Three of the 
worst weeds we have in New Zealand—viz., Sorrel (Ru- 
mex acetosella), one of the Couch-grasses (Poa pratensis), 
and the Creeping Thistle* (Carduus arvensis)—spread by 
means of their long slender rhizomes. Hence the difficulty of 
eradicating these plants, as every little bit of stem bearing a 
bud is capable of reproducing a new plant. 
The tubert is a short, more or less rounded, and thickened 
stem, bearing buds on its surface. The most familiar example 
is found in the potato. It serves chiefly as a mode of repro- 
duction, there being always a quantity of nutritive material 
(most commonly starch) stored up in it for the new plant. 
_ The bulb} is a short thick stem, consisting of a central 
* This plant is popularly but incorrectly known as the Californian or 
Canadian thistle. It is common enough in Canada, and perhaps 12 
California, but was introduced into America from Europe, where it 18 
indigenous, and from whence it was brought to this colony. 
t Lat. tuber, a lamp or excrescence. _ 
? Lat. bulbus, a globular root, an onion. 
