equently favors the
storage of sugars instead of starches. Thus, in temperate zones, among
aquatic, or moisture-loving plants, those species which hibernate during
the winter at the bottom of lakes or ponds and are killed by temperatures
below freezing, store starch and no sugar; while in the same ponds, the
species whose storage organs pass the winter above the level of the water
and can withstand temperatures as low as -7 deg. C. contain sugar during
the winter months, even if they contain starch during warmer periods.
Similarly, sugars often appear in the leaves and stems of conifers during
the winter months, only to disappear, or be replaced by starch, when spring
approaches. This same phenomenon is noticeable in arctic plants, which
generally contain but small proportions of starch and relatively large
amounts of sugars.

Similarly, the phenomenon of the turning sweet of potatoes when exposed to
low temperatures has often been noted. The change of the starch in potato
tubers to sugar is most rapid at the temperature of 0 deg. C., and ceases
at 7 deg., or above. Also, if potatoes in which the maximum amount of
sugar is present (not over one-sixth of the total starch can be converted
into sugar) are exposed to a higher temperature the sugar soon disappears.

In general, however, it may be said that each particular species of plant
has its own particular preference for a specific carbohydrate as its
reserve food material, and elaborates the proper enzymes to make it
possible to utilize this particular carbohydrate for its metabolic needs.

Again, the question as to whether the storage of energy-producing materials
for the use of the next generation shall be in the form of carbohydrates or
of fats seems to be definitely connected with the size of the seed, and the
consequent available storage space (see page 138). Animals habitually use
the space-conserving form of fats for their energy-storage, while plants
more commonly use carbohydrates for this purpose, except in the case of
those small seeds in which sufficient energy cannot be stored in
carbohydrate form to develop the young seedling to the point where it can
manufacture its own food. As a general rule, nuts, which contain the embryo
of slow-growing seedlings, and need large proportions of energy reserve,
are characteristically _oily_ instead of _starchy_ in type.

But, aside from temperature reactions and space requirements, there is no
law which has yet been