CHE REFERENCE LIBRARY - Pole Shift Survival Information

20 THE NURSERY-MANUAL
the double purpose of preventing too great heat and too rapid
evaporation. Various covered seed-boxes are uF?d for the
purpose of maintaining approximately the required ‘L?mpera-
ture, but as they are oftener employed in bud-propagation,
they are discussed in that connection (Chapter V).
Bottom heat is helpful to germination in most seeds, but,
except in the case of certain tropical species, it should not be
strong It is a common practice to place seed-boxes on moderately
cool pipes under benches in a greenhouse. Seeds of
hardy annuals and perennials do not. require bottom heat,
although they may be benefited by it. If the earth in seedbeds
should become too cool, watering with warm or tepid
water may be helpful.
It is impossible to give rules for the proper temperature for
different kinds of seeds. In general, it may be said that seeds
germinate most rapidly at a temperature a few degrees above
that required for the best development of the plant itself.
Seeds of hardy plants require a temperature of 50” to 70”,
conservatory plants GO” to SO” and tropical or stove plants
75” to 95>“. The plantlets should be removed from these
highest temperatures, as a rule, as soon as germination is
completed.
In outdoor culture, depth of planting has a direct relation
to temperature. Seeds may be planted deeper late in the
season than early when the soil is cold and damp. Deep
planting probably as often kills seeds because of the absence
of sufficient warmth as from the lack of oxygen or the great
depth of earth through which the plantlet is unable to push.
The influence that light exerts on germination is not definitely
understood for all horticultural seeds. It is known, however,
that seeds will often germinate in full sunlight, if the lproper