THE POTATO TUBER. 
59 
(4) The comparative impermeability of the skin 
of the tuber to water and certain salts and 
acids in solution. 
The Morphology of the Potato Tuber. 
The principal morphological characters as presented 
by an ordinary well-grown tuber are not difficult to de- 
termine. It may at once be asserted that it represents a 
short, much thickened part of an underground structure, 
which, on casual observation, would probably be regarded 
as a root. If it be desired to express this in formal botanical 
terminology, then this underground apparently root-like 
structure is designated by the word rhizome, and its enlarged 
(and in this case terminal) portion by the word tuber. In 
point of fact it would be difficult to find an object which 
more aptly illustrates the botanical definition of a tuber 
tlian the potato. That the rhizome is actually an under- 
ground stem, and not a root, and the fact that tubers are 
borne on rhizomes and not on roots, is rendered apparent 
by a comparative wealth of convincing evidence, each item 
of which may, if desired, be readily subjected to independent 
verification. In the first place we have the fact that the 
structures bearing the tubers arise from buds in the axils 
of the vegetative leaves, and hence they are regarded as 
branches of the stem. The rhizome, it may be further stated, 
in order to obviate any misapprehension in regard to its 
origin, a rises exogenously, that is to say, from the super- 
ficial layers lying towards the peripheral or cortical portion 
of the stem, which it will be recalled arises during the course 
of its development from the periblem. Another point of 
difference between rhizomes and roots which may be noted 
is that the rhizome is provided with an apical bud ; a root, 
on the contrary, lias a root cap. Again, rhizomes bear 
scale leaves at distinct nodes. Roots never bear leaves. 
Finally, the internal anatomy of the rhizome is that of a 
stem. 
We may now turn to the gross structure or external 
morphology of the tuber. Two well-marked regions are 
distinguishable : the “ heel ” or basal portion (Fig. ia), 
to which the withered remains of a part of the rhizome is 
usually found attached, and the “ rose ” (Fig. 16) end or 
apical portion. At the surface of the tuber are the “ eyes ” 
(Fig. id) arranged in a spiral. These are invariably more 
crow'ded towards its apical or " rose ” end. Each “ eye ” 
appears as a collection of buds lying in a depression which 
usually, in very young tubers, may be readily identified 
as the axil of a scale-leaf. As the tuber develops these 
scale leaves wither and are lost to view, so that in mature 
