RETARDING VEGETATION. 



395 



over-covering, and yet, for the reasons wliich we have just mentioned, it 

 would be very unsafe to leave a hotbed uncovered during any of the nights 

 of winter or early spring ; though later in the season, or where plate-glass 

 is used, covering at night might be dispensed with. The covering should 

 not be drawn over the linings so as to confine the steam ; which in that 

 case would find its way into the frame to the injury of the plants. The 

 temperature and moisture to be kept up in hotbeds vary with the kinds of 

 plants, and the object in view. 



§ XVII. Retarding Vegetation. 

 848. The different modes of retarding vegetation being in many cases 

 the opposite of those for its acceleration, the subject may be similarly 

 arranged. As on the south side of ridges of ground, in the direction of east 

 and west, plants are accelerated by meeting the rays of the sun at a larger 

 angle, so on the north side of such ridges, as well as on the north side of 

 walls and hedges, they will be retarded by the exclusion of the sun's direct 

 influence. Opaque coverings, put on in winter or in early spring, are also 

 effective, more especially when of some thickness, by excluding the stimulus 

 of light, and presenting a thicker mass to be penetrated by atmospheric heat. 

 Thus herbaceous perennials, such as asparagus, rhubarb, sea-kale, and other 

 plants which do not retain their leaves during winter, may, by a thick 

 covering of leaves or litter put on in January, when the soil is at the coldest, 

 be prevented from vegetating for a week or a month later than the same 

 plants on a surface sloping to the south, without any covering, and with the 

 soil dry and loosened about the collars of the plants. The production of 

 blossoms and fruit may in many cases be retarded by taking off the flower- 

 buds at their first appearance in spring or early summer, as is often done 

 with roses, strawberries, and raspberries, which when so treated flower and 

 fruit a second time in the autumn. Even the common hardy fruit-trees, — 

 the apple, pear, cherry, &:c., — when so treated will blossom and set their 

 fruit a second time in the same year, but it will not ripen from the length 

 of time required. Currants and gooseberries, and even pears and apples on 

 dwarfs, are preserved on the trees till Christmas, by matting them over ; 

 and the season of wall-fruits and of grapes in hothouses is prolonged by 

 excluding the sun and preserving the air dry. In general, all exogenous 

 perennial herbaceous plants, when cut over as soon as their flower-buds are 

 formed in spring, will spring up again and produce flowers a second time in 

 autumn ; but this does not happen with endogens, excepting in the case of 

 grasses and a few other plants. Retarding no less than accelerating may be 

 effected by changing the habits of plants ; and thus, as plants which have 

 vegetated early one season are likely also to vegetate early the season fol- 

 lowing, so plants which have continued to grow late in autumn one year 

 will be later in vegetating in the following spring, and continue to grow later 

 in the autumn. There is a considerable difference in the natural earliness 

 and lateness of vegetation in all plants of the same species or variety raised 

 from seed, and hence, early and late varieties may always be procured by 

 selection from the bed of seedlings. By this means have been obtained all 

 the earliest and latest varieties in cultivation both in fields and gardens. 

 Seeds or plants procured from cold and late soils and situations, and brought 

 to earlier ones, continue for a time to be late from habit, and the contrary ; 

 and hence the practice of farmers in cold, late districts procuring their seed- 



