Breeding of long standing, both instinctive and purposive, has helped to create the rich collection of vegetative species in its present form.
Experience shows that forcing produces best results principally with those cultivars that have been developed especially for this purpose.
It is true that in the practice of vegetable growing there exist numerous and good cultivars with a large area of distribution.
They are grown both in the open and in greenhouses.
This, however, is not the best solution of the problem.
The grower, orientated toward forcing of vegetable crops wants the results of his labour to be well paid.
This condition may be met when specially created cultivars are grown in the greenhouse under appropriate conditions and technology.
Precisely this requires greater attention to be paid to the problems of breeding.
Today, to the traditional greenhouse vegetable crops such as cucumbers, tomatoes, peppers, lettuce, kohlrabi are added some other crops, the species collection of which depends on the possibilities of their breeding and on their demand on the market.
Recently, certain progress is noted not only in tomatoes but also in cucumbers.
Cucumbers have acquired great economic importance as greenhouse crop.
The main reason for this is that they are a culture which has been especially bred for forcing.
It may be said that on the basis of this experience a new technology of greenhouse and hotbed vegetable growing has been created.
Far back in history, the Roman emperor Tiberius was very fond of cucumbers and wanted to have them throughout the year.
That is why cucumbers were artificially grown in special cases, carefully topdressed and protected from external influences - the beginning of the present greenhouse and hotbed vegetable growing.
The specific conditions which the grower creates for the development of greenhouse and hotbed crops call for special breeding.
Protected crops receive all basic nutrients in the optimal degree.
That is why it is necessary to develop such cultivars that not only can thrive and develop well under these conditions but also may use them to the best advantage in a brief period of time.
Here more than anywhere else the principle breeding is valid to be conducted in the conditions under which the crop will best be grown in practice.
It is further necessary to call attention to the problem of plant resistance.
It is of great importance for cultivars intended for forcing in some respects even more important than for cultivars grown under ordinary conditions.
The optimal conditions created for forced crops is the reason for these plants to be rather susceptible.
Analogous phenomena are frequently met with in nature and they show that excessively productive organisms,