STEMS 65 
of the water-lilies, from which arise the leaves with large 
floating blades (pads). Therefore, a rhizome does not nec- 
essarily bear only scale leaves, but may develop also leaves 
that become aerial; and in that case they are usually 
large. It is evident that in plants possessing rhizomes 
the subterranean stems are perennial, while the aerial parts 
may be annual. 
(5) Tubers.—In some plants the ends of underground 
stems become very much enlarged for food storage. These 
enlargements are called tubers, the best-known illustration 
being the common potato (Fig. 64). That it is a stem 
structure is evident from the fact that it bears very much 
Fic. 64.—Potato tuber showing eyes (scale leaves and axillary buds). 
reduced leaves, in the axils of which are buds, the so-called 
“eyes.”’ Abnormally developed potatoes often show the 
shoot character of the tuber very plainly, and in the case 
of potatoes sprouting it is evident that the eyes have de- 
veloped into branches. In planting potatoes, advantage is 
taken of the fact that any node placed in proper conditions 
may strike root and put out a branch. Since the eyes are 
branch buds standing at nodes, and any piece of the 
potato containing a bud is able to produce a new plant, it is 
customary to cut the potato into pieces, being careful that 
each piece contains one or more eyes. Heaping up the 
soil (hilling up) about the base of the potato plant induces 
the formation of more of the subterranean, tuber-bearing 
