1891.] ESSAYS. 49 



Great caution should be used at the first transplanting from 

 the seed bed not to use plants that are infested at all with the 

 green aphis or lettuce louse. If by any means even one louse is 

 introduced into your frames, you have a hard enemy to tight, for 

 they multiply so fast that if they do not ruin the first crop, they 

 are almost sure to destroy the second winter crop in those frames. 

 The only effectual way to destroy them, when they get on the 

 plants, is by fumigating with tobacco smoke while the plants are 

 comparatively small, before the foliage is dense enough to pre- 

 vent the smoke from penetrating to every hiding place. The 

 fumigation of lettuce in the hot-bed frame is much more difficult 

 than it is when growing in the green-house. Indeed, fumigating 

 the frames is seldom attempted. Weekly or semi-weekly fumiga- 

 tion is needed in the green-house when the plants are young. 



But the real difficulty in lettuce growing under glass begins 

 when the lettuce is planted upon bottom heat, or in the green- 

 house, after the weather is cold enough to close your house tight 

 and start your fires. Here the necessary heat, moisture, and 

 needed change of air must be produced artificially, and it re- 

 quires almost infinite skill to understand, and to regulate all 

 these, and adapt them to the varying light and change of outside 

 temperature. The temperature must be lowered just as soon as 

 the sun ceases to shine, even during daylight, or the plants will 

 soon become unhealthy. 



As the plants begin to head in and, especially, just before they 

 have attained the proper size and .solidity for market, the black 

 rot in the head, mildew and various other unnamed and unnama- 

 ble diseases, are liable to manifest themselves, that greatly injure 

 and sometimes utterly destroy the crop. 



Night Temperatuke. 

 It used to be thought that the night temperature should be 

 45° and from that up to 50°. But experience has convinced 

 many, and our experiment station has verified this opinion that a 

 lower night temperature is necessary to avoid mildew. 40° or 

 less is now regarded as high enough for healthy growth, especial- 

 ly after a cloudy day. Tlie day temperature in clear weather 

 should be allowed to go up to 60° or 70°. If the weather is 

 cloudy the day temperature should not go above 55°, 



