SELECTING AND IMPROVING PLANTS IN CULTURE. 407 



same, or of allied kinds ; and in the latter something more should be done. 

 The reasons are, that in both cases the farina of adjoining flowers of the same 

 kind is in all probability floating in the atmosphere, and will adhere to 

 whatever stigmas of its own species it may light on ; and secondly, that bees 

 and other insects which frequent flowers carry off the pollen from one to 

 another, and thus produce accidental cross-fecundation, which would render 

 nugatory that which was attempted by art. The only mode to guard against 

 pollen floating in the atmosphere is by placing the plants from all others of 

 the same kind, though what distance is required is uncertain. For the 

 crucifera3 generally most space is required ; varieties of cabbages and turnips 

 having been adulterated when at the distance of upwards of a mile, in an 

 open country and in the direction of the prevailing winds. To guard against 

 the effects of bees and other insects, the blossoms when selected and fecun- 

 dated by art may be surrounded by coarse gauze, or inclosed in a case of 

 glass, till the blossom begins to fade. To strengthen the embryo seeds, the 

 plant may be pruned in such a manner as to throw an extra share of sap 

 into the branch, stem, or pedicel on which the flower is situated. Thus, if 

 the fecundated flower form part of a spike, the upper part of the spike may 

 be cut off; a corymb or an umbel may be thinned out ; the suckers may be 

 taken from a sucker-bearing plant, such as the raspberry ; the runners from 

 the strawberry ; the offsets from a bulb, the tubers from a potato, and so 

 forth. 



867. Fixing and rendering permanent the variety produced is effected, in 

 general, by one or other of the modes of propagation by division (551). 

 Improved varieties of fruit-trees are generally perpetuated by grafting; fruit- 

 shrubs, such as the gooseberry, by cuttings ; perennials, by division, offsets, 

 or suckers, &c. ; improved annuals and biennials, and some perennials, are 

 perpetuated by seeds, which forms an exception to the general rule. What 

 \ve have already advanced on this subject in the paragraph last quoted ren- 

 ders it unnecessary to dwell on it here, farther than to notice a practice, the 

 result of the experience of cultivators, the rationale of which it is difficult to 

 explain. This is the transplantation of culinary biennials, such as the turnip, 

 carrot, parsnep, beet, cabbage, cauliflower, onion, and many such plants, after 

 they are full grown, previously to their being allowed to send up their flower- 

 stems. By this practice the variety is said to be prevented from degene- 

 rating ; and if so, it may probably be on account of the greater part of the 

 nourishment to the seeds being furnished by the store laid up in the plant, 

 and but only a small portion taken from the soil. It is certain that trans- 

 planted plants do not produce nearly so much seed as they would have done 

 if not transplanted ; and it is equally certain that in the case of the turnip, 

 when the bulb is of a moderate size, and even small rather than large, much 

 stronger flower-stems are sent up, and more seed produced, than when it is 

 large. The reason probably is that the roots below the unswelled bulb are 

 stronger, not having yet fulfilled their functions, and hence are enabled to 

 draw a larger proportion of nourishment from the soil. 



868. The production of double flowers is a subject not yet thoroughly 

 understood by physiologists. As double flowers are seldom found in a wild 

 state, they appear to be the result of culture, and yet there is scarcely any 

 well-authenticated instance of culture having produced them. It is certain, 

 however, that double flowers degenerate into single ones when culture is 



