52 DREER’S VEGETABLES UNDER GLASS. 
gone so far and become so famous that it must be separately 
classified. It is the hot bed system with the manure replaced 
by hot water or steam, and with the movable sashes replaced 
by great glass structures of which nobody dreamed a gener- 
ation ago. 
Lettuce is pre-eminently the most profitable of the forc- 
ing house crops, though other popular vegetables and fruits 
are now rapidly coming into favor. It is lettuce, however, 
to which the greatest winter acreage is given. ‘Two crops of 
lettuce are always grown and sometimes three crops, and the 
glass then goes to cucumbers, tomatoes, &c. But lettuce has 
first place, as will be described in the paragraphs about 
Boston methods. 
Oren AIR CULTURE. The fifth of the great methods 
of lettuce culture, the out-of-doors way, is not within the 
range of this little book. In passing, it may be well to say 
that lettuce may go into the open ground as soon as the soil 
can be worked. Young plants ought to be ready in the 
earliest spring, from seed sown in frames or from wintered- 
over plants. 
If the plants are set out or sowed in the open ground in 
rows ten inches apart, and allowed to stand eight inches apart 
in the rows, there will be room for all the necessary cultural 
operations and for the subsequent development of the heads. - 
The richest and mellowest soil is demanded for successful let- 
tuce culture, whether under glass or in the open air. 
VARIETIES. The varieties of lettuce in favor among 
gardeners at this time are very numerous, and we can 
merely give a few suggestions. Much depends upon soil and 
situation. 
For ridge culture in the neighborhood of Philadelphia 
the sort that is most widely popular is the Early White Cab- 
bage or White Butter. It is almost entirely hardy. Early 
