LETTUCE AND RADISHES 315 



serious outbreaks of disease are likely to occur. The damp atmos- 

 phere and dark weather are not only favorable to the develop- 

 ment of the fungi causing disease, but also encourage a sappy 

 growth in the plants which is more susceptible to disease than a 

 harder, more normal growth. In cloudy weather it is necessary 

 to water sparingly and hold the temperature rather low if trouble 

 is to be avoided. 



Certain insects also thrive better in the greenhouse than out- 

 of-doors, and an almost continual warfare must be waged against 

 them if the crops are to be fully protected. In fact, some growers 

 practice fumigation of their houses regularly, at intervals of about 

 a week, rather than ever give the plant lice and other insects a 

 chance to become seriously abundant. 



Thus, in greenhouse vegetable growing, constant attention 

 must be given to the regulation of heat, ventilating, watering, 

 and the control of insects and diseases. Yet, when properly 

 managed, the crop in a greenhouse is under much more complete 

 control than an outdoor crop. 



Selection of Crops for Forcing. — Greenhouse vegetable grow- 

 ing is the most intensive type of vegetable production, and only 

 those crops which yield a product of high value per square foot 

 of space occupied can be profitably grown under glass. It is also 

 true that certain crops adapt themselves more readily to green- 

 house conditions than others. There are also special forcing strains 

 of nearly all the vegetables commonly grown under glass. These 

 have, in many instances, been developed under greenhouse condi- 

 tions, and are better adapted to such conditions than are the 

 varieties and strains commonly grown out-of-doors. Furthermore, 

 some of the best forcing strains are of little value for outdoor 

 culture, owing to the difference in conditions, though some sorts 

 are well adapted to both uses. The point that must be kept in 

 mind is that, in order to succeed in growing vegetables under glass, 

 it is necessary to select varieties and strains that are adapted to 

 the purpose. Such varieties are usually designated distinctly in 

 seed catalogues. 



Lettuce and Radishes. — The leading forcing-house crops are 

 lettuce, radishes, cucumbers, and tomatoes. Of these, lettuce is 

 by far the most important. Both radishes and lettuce make a 

 very rapid growth, so that, in spite of the short days of winter, 

 several crops can be grown in succession in the same house in one 

 season. In order to economize space, the lettuce is usually started 



