Most of the crops mentioned above can also reproduce sexually by producing flowers
and seeds and thus have two ways in which to reproduce themselves. Onions for
example can be either grown vegetatively from bulbs (sets) or sexually from seed.
Clone is the name given to a group of plants (or animals) that are all genetically
identical as a result of having been propagated vegetatively (asexually).
These terms describe the two ways in which annual plants produce flowers and are
mainly used when describing legumes.
Different varieties of the same species may differ in this respect. With soybeans,
cowpeas and tomatoes for example there are both determinate and indeterminate
varieties.
started to flower. The terminal or central flower is the first to open. The main advantage,
especially for mechanised harvesting, is that almost all of the grain matures at about the
same time.
started. The terminal flower is the last to open. They tend to have fewer branches than
determinate types, and are normally grown in areas with short day length ie where
flowering begins early on in the life cycle of the plant. They can be very useful in
subsistence agriculture because they produce a fairly regular food supply, harvested by
hand, over a long period of time. But with mechanical harvesting they can cause
problems because on any given day only a proportion of the seed will be mature and
ready for harvest. The rest of the grain will be either immature, and too moist, or it will
be lost because it has shattered ie previously matured and fallen to the ground.
Many plants, including most of the legumes, are day length sensitive. This means that
their reproductive stage, ie flowering, is triggered by their response to day length, the
number of hours sunshine in a 24 hour cycle.
Photoperiodism in plants can be defined as the effect on flowering of the relative
lengths of day and night. Plants respond to day length in one of three ways:
- Short-day plants stop flowering as soon as the day length has passed a critical
value:
Examples: rice, most soybean varieties, maize, sorghum and millet. They generally
require only 8 or 9 hours of sunlight (ie a dark period of at least 12 hours) in order to
flower;
- Long-day plants start to flower soon after the day length has passed a critical
value:
Examples: carrot, pea, lentils, oats and most varieties of wheat. The day length on the
Equator is normally not long enough for these species to flower;
42 TONY WINCH
Determinate varieties of plants increase in growth very little, if at all, after they have
Indeterminate varieties continue to grow and increase in height after flowering has
i Determinate/Indeterminate
1Ej. Day length /Photoperiodism
