RETARDING VEGETATION. 363 



covering, and yet, for the reasons 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. 



Retarding Vegetation. 



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. 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. Ketarding 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 following, so plants which have 

 continued to grow late in autumn one year, will be later in vege- 

 tating in the following spring, and continue to grow later in the 

 autumn. There is a considerable difference in the natural earli- 

 ness 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 



