866 RESTING VEGETATION. 



compared with that of the day, can only be determined by experience ; 

 because plants under glass are so far removed from plants in the free 

 air, that the same difference which takes place in the latter case may 

 not in the former case be advisable. Nevertheless it is clear from the 

 experience of gardeners that a very great fall during the night is 

 seldom or never attended with bad effects, provided there has been 

 sufficient heat and light during the day. Much of the evil of a high 

 temperature during night, especially where opaque coverings are used, 

 must be owing to the absence of light. A fall of from 10 to 15 at 

 night under the maximum maintained by day, would seldom be inju- 

 rious, and as a rule might be safely and advantageously adopted. 



Double glass roofs would evidently form the least objectionable 

 nightly covering to plant structures of every kind ; and next to these 

 the use of plate-glass, instead of common crown glass, as from the 

 much greater thickness of the former i'ar less heat would be allowed 

 to escape by conduction. The use of plate-glass in cucumber and melon 

 frames, and also in greenhouses and forcing-houses, has of late years 

 been adopted by several persons, and the glass being much less liable to be 

 broken, and requiring no covering during night, it is found to be on the 

 whole more economical than common glass, and much better for the plants. 



The annual resting of plants is effected, as we have seen, either by 

 cold or by dryness, and both these causes can be imitated in a state of 

 culture, either separately or combined. Plants in the open garden 

 may be safely left to the influence of the seasons ; but half-hardy 

 plants against walls, or in borders by themselves, may be brought to 

 a state of rest by thatching the ground so as to prevent what rain may 

 fall on it from sinking in ; the lateral supplies being cut off by sur- 

 face gutters or underground drains. The supply of sap by the roots 

 being thus reduced, growth will gradually cease, and the parts will be 

 matured, and at once enabled to resist the winter and vegetate with 

 redoubled vigour the following spring. It may be observed here that 

 the shoots of a tree which is to be protected from frost during winter, 

 do not require Lo be ripened to the same degree as shoots which are 

 to be exposed to the action of frost in the free atmosphere ; because 

 buds, like seeds, will vegetate provided the embryo be formed, even 

 though they should not be matured. Plants which have been forced 

 have their period of rest brought on naturally by the maturation of 

 the plant ; and hence peach-trees which have been forced, have almost 

 always better ripened wood, containing more blossom-buds, than 

 peach-trees on the open walls. In the case of peach-houses, vineries, 

 .&c., by withholding water and applying dry heat, maturation is easily 

 effected. Greenhouse plants, such as natives of the Cape of Good 

 Hope and Australia, are brought to a state of rest, partly by lower- 

 ing the temperature of the greenhouse and partly by withholding 

 water. The last mode is that which is most to be depended on, 

 because in most greenhouses there are some plants in flower at every 

 period of the year, and for these a greater degree of heat must be kept 

 up than would merely suffice for throwing greenhouse plants into a 

 state of rest. All tropical plants are brought to a state of repose by 



