- Varieties with a short growth period can often be planted at a time of the year
which avoids periods which are difficult for plant growth, such as in the dry season
or when insect or disease attack is likely to happen.
So we can see that farmers who decide to grow an early variety, and who probably
accept a lower yield as a result, often make a good decision, especially if they can then
immediately plant a second crop which would not have been possible if they had planted
a later but higher yielding variety.
Plants have a number of elaborate ways of adapting to their environment, one of
which is by changing their growth period. In general, plants which are growing in
unfavourable conditions (too hot or dry, or in infertile soils for example) will grow
faster, ie will have shorter growth periods, than plants growing in more favourable
conditions. The plant is stimulated to reproduce itself, by producing at least a few seeds,
as a result of the unfavourable conditions—if the plant delayed maturity any longer it
might die before it could produce any seed at all. So the plant shortens its growth period
and thus becomes more likely to succeed in reproducing itself.
If the plant is grown in more favourable conditions its growth period will probably
be longer. These conditions may be changed either by man or by a change in climatic or
field conditions. For example, if a variety of a crop is taken from one altitude to a higher
altitude its growth period may be reduced. Or if heavy applications of fertiliser or
irrigation water are supplied, the growth period may be extended because the plants are
then provided with more favourable conditions.
Plant breeders spend a lot of time and energy developing varieties that mature early
ie which have a short growth period. The challenge facing plant breeders is how to
incorporate this precocious behaviour with acceptable yields.
Late maturing varieties, with longer growth periods, have their own advantages, such
as larger and more dependable yields, especially when neither the shortage of land nor
the immediate supply of food is a serious concern for the farmer.
A tuber is an enlarged, swollen tip of an underground stem (or rhizome). Tubers have
buds, or nodes, in the axils of rudimentary leaves or scales. They typically have a high
starch content. Examples: Irish potato and Jerusalem artichoke (Helianthus tuberosus).
Tubers contain stored food, mainly starch, and are used by food growers in the
vegetative propagation of some crops. They are often confused with “roots”—the Irish
potato, which actually produces tubers, is normally called a root crop, though
technically they produce tubers and not roots.
The true root crops are those where the root itself, and part of the hypocotyl, is
enlarged and which do not form leaf buds on the root. Examples: carrots, turnips,
Reproduction”, page 40.
“Vegetative44 TONY WINCH
cassava and sweet potato. Tubers are also described in 1Eh.